Why was the prodigal son spoken harshly of by his elder brother?

One commentary portrays a Christian's relationship with the law.

The elder son in this parable portrays the Pharisees and scribes. In 15:3 we see that the Lord spoke these parables to the Pharisees and scribes in the presence of the tax collectors and sinners. The religious ones must have been unhappy, but the tax collectors and typical sinners must have been filled with rejoicing. The Pharisees and scribes, however, complained that the situation portrayed in these parables was not fair.

Watchman Nee illustrates:

The story of the prodigal son gives a supreme illustration of the way to please God. The father said, "We had to be merry and rejoice" (Luke 15:32). The matter that was supremely joyful to the father's heart was not the elder brother's incessant toiling for the father, but the younger brother's willingness to let the father do everything for him. What was pleasing was not the elder brother who wanted to be the giver, but the younger brother who was willing to be the receiver. When the prodigal returned home after wasting all of his substance on riotous living, the father did not have a word of rebuke concerning the waste or a word of inquiry regarding the substance. He did not sorrow over all that was spent; he only rejoiced that the prodigal's return afforded him the opportunity to spend much more. God is so wealthy that His chief delight is to give; His treasure-stores are so full that it is painful to Him when we refuse Him an opportunity to lavish His treasures upon us. It was the father's joy that he found a willing applicant in the prodigal son for the robe, ring, shoes, and feast; it was his sorrow that he found no such application in the elder brother. It is a grief to the heart of God when we try to give things to Him, because He is so rich. It is a joy when we simply allow Him to give and give to us. It grieves Him when we try to do things for Him, because He is so able. He longs for us to simply allow Him to do everything. He wants to be the Giver and the Doer eternally. If only we saw how rich and how great He is, we would leave all the giving and doing to Him.

The prodigal son was not exempted from this either.

When the prodigal son considered his situation, he may have asked himself, "Why am I doing this? My father is very rich, so why should I perish here in famine?" This is a sinner’s repentance. However, the concept of a sinner after his repentance is to return home to work.

Therefore, the prodigal son went on to say,

"I will rise up and go to my father, and I will say to him, Father, I have sinned against heaven and before you. I am no longer worthy to be called your son; make me like one of your hired servants" (vv. 18-19).

Then he rose up and went to his father to speak according to what he had prepared. However, the father did not want to hear what he had to say, so before the son could finish his speaking, the father interrupted him and told the servants, “Bring out quickly the best robe and put it on him, and put a ring on his hand and sandals on his feet. And bring the fattened calf; slaughter it, and let us eat and be merry” (vv. 22-23). The fattened calf signifies Christ, who is God. God in Christ has become the fattened calf for the enjoyment of the repentant and returning prodigal sons. To us, this is the jubilee.

Therefore, Luke 15:11-32 is an illustration of the jubilee proclaimed in Luke 4:18-19. The prodigal son sold his possession and himself. One day he returned to his possession and his father’s house. That was a jubilee, a liberation, and everything became pleasant and satisfying. In the father’s house there was only enjoyment with eating and drinking; there was no labor.

This corresponds to Leviticus 25:11, which says that the people were neither to sow or reap in the year of the jubilee; they should only eat and enjoy. Furthermore, they could only eat of the produce directly from the field. This means that they ate what God supplied without the need of their own labor. Similarly, the father in Luke 15 did not listen to what the son had to say about being a hired servant. Instead, the father desired to give the son the fattened calf for him to eat and enjoy. No one is unworthy; rather, all are worthy because God says, “I have accepted you.” The jubilee is the age, the time, of God’s acceptance, indicated by the father’s acceptance of the prodigal son in Luke 15.

So, the elder brother who was "positionally" in the Father's house but never "experientially" enjoyed the Father's riches. While the prodigal one, who was a sinner, did.

EDIT:

Why was the prodigal son spoken harshly of by his elder brother?

Just as it was portrayed by in Genesis 21 (due to Paul's allegorizing the two women in Galations), it was not the birth of Isaac that stirred up trouble; it was his growth. When Isaac was born, Hagar and her son Ishmael were not bothered very much. But after Isaac had grown up, Ishmael began to mock him (v. 9). In the biblical sense, this means that Ishmael was persecuting Isaac. God even counted Ishmael's persecution of Isaac as the beginning of the four-hundred-year persecution of His people (15:13; Acts 7:6). As Paul says in Galatians 4:29, "But as then he that was born according to flesh persecuted him that was born according to Spirit, so also it is now."

Just like the religious law-keepers people cared only for the ritual of Sabbath-keeping in John chapter 5; they had no concern for the sick man's condition. The law-keeping religion could not give life to the impotent man. When Christ enlivened this man with life, that religion persecuted Him because He did it on the Sabbath (5:16). That religion cared for the keeping of its Sabbath at the sacrifice of the impotent man's rest. But Christ cared for the man’s rest at the sacrifice of their Sabbath-keeping. This surely offended that religion.

There are many cases where religious law-keeping portrayed in the Gospels (fasting, Sabbath, eating, regulations, ordinances, and rituals etc.) exposes religious law-keeping's focus on religious regulations rather than those in real need.

Was Jesus intending to bring out the degeneration of relations over property and wealth by 'making' the elder son speak harshly of the younger one?

No. Rather, the proclamation of the jubilee in Luke 4 governs the central thought of the whole Gospel of Luke. In Ephesians 1:13-14, Paul says,

In whom you also, having heard the word of the truth, the gospel of your salvation, in Him also believing, you were sealed with the Holy Spirit of the promise, who is the pledge of our inheritance unto the redemption of the acquired possession, to the praise of His glory.

What does it mean to be saved? To be saved is to return to our inheritance, to return to God, to come back to God and enjoy Him anew as our possession (Psalm 16:5). God is our inheritance, and after we are saved, the Spirit of God is in us as the pledge, the guarantee, the proof, and the security of our inheritance (Ephesians 1:13-14).

According to spiritual significance, this story depicts a fallen man who completely lost his possession in the house of God the Father. He left his own possession and sold himself as a slave [to sin, world etc.].

Man has lost God and is without God because of the fall. Therefore, when the Bible speaks of the year of jubilee, the first thing it shows is that man must be returned to God. According to the Old Testament type, when the year of jubilee came, a person who had sold himself into slavery was returned to his own possession and to his own family to be reunited with his relatives, and at the same time he was also released from the yoke of slavery and was no longer a slave.

In the New Testament age, in Luke 4, the Lord spoke of the condition of three kinds of people.

Verse 18 says:

The Spirit of the Lord is upon Me, because He has anointed Me to announce the gospel to the poor; He has sent Me to proclaim release to the captives, and recovery of sight to the blind, to send away in release those who are oppressed.”

The three categories of people in this verse are the poor [To be without God is to be really poor], the captives, and the oppressed. On the one hand, these are three kinds of people, but on the other hand, they are three human conditions. When we lost God, we became poor, and the result of poverty was that we were captured. Then after being captured, we were oppressed. This illustrated the prodigal son's condition after he left the Father's house.

Now the tax collectors and sinners were all drawing near to hear him. And the Pharisees and the scribes grumbled, saying, “This man receives sinners and eats with them.” So he told them this parable: “What man of you, having a hundred sheep, if he has lost one of them, does not leave the ninety-nine in the open country, and go after the one that is lost, until he finds it? And when he has found it, he lays it on his shoulders, rejoicing. And when he comes home, he calls together his friends and his neighbors, saying to them, ‘Rejoice with me, for I have found my sheep that was lost.’ Just so, I tell you, there will be more joy in heaven over one sinner who repents than over ninety-nine righteous persons who need no repentance. “Or what woman, having ten silver coins, if she loses one coin, does not light a lamp and sweep the house and seek diligently until she finds it? And when she has found it, she calls together her friends and neighbors, saying, ‘Rejoice with me, for I have found the coin that I had lost.’ Just so, I tell you, there is joy before the angels of God over one sinner who repents.” And he said, “There was a man who had two sons. And the younger of them said to his father, ‘Father, give me the share of property that is coming to me.’ And he divided his property between them. Not many days later, the younger son gathered all he had and took a journey into a far country, and there he squandered his property in reckless living. And when he had spent everything, a severe famine arose in that country, and he began to be in need. So he went and hired himself out to one of the citizens of that country, who sent him into his fields to feed pigs. And he was longing to be fed with the pods that the pigs ate, and no one gave him anything. “But when he came to himself, he said, ‘How many of my father’s hired servants have more than enough bread, but I perish here with hunger! I will arise and go to my father, and I will say to him, “Father, I have sinned against heaven and before you. I am no longer worthy to be called your son. Treat me as one of your hired servants.”’ And he arose and came to his father. But while he was still a long way off, his father saw him and felt compassion, and ran and embraced him and kissed him. And the son said to him, ‘Father, I have sinned against heaven and before you. I am no longer worthy to be called your son.’ But the father said to his servants, ‘Bring quickly the best robe, and put it on him, and put a ring on his hand, and shoes on his feet. And bring the fattened calf and kill it, and let us eat and celebrate. For this my son was dead, and is alive again; he was lost, and is found.’ And they began to celebrate. “Now his older son was in the field, and as he came and drew near to the house, he heard music and dancing. And he called one of the servants and asked what these things meant. And he said to him, ‘Your brother has come, and your father has killed the fattened calf, because he has received him back safe and sound.’ But he was angry and refused to go in. His father came out and entreated him, but he answered his father, ‘Look, these many years I have served you, and I never disobeyed your command, yet you never gave me a young goat, that I might celebrate with my friends. But when this son of yours came, who has devoured your property with prostitutes, you killed the fattened calf for him!’ And he said to him, ‘Son, you are always with me, and all that is mine is yours. It was fitting to celebrate and be glad, for this your brother was dead, and is alive; he was lost, and is found.’” Luke 15:1-32 ESV

Luke 15 is a set of three of the same Parable but given from different angles. The Parable of the Lost Sheep, Parable of the Lost Coin, and Parable of the Two Sons.

Contrary to popular understanding, the Parables are about "Hearers of the Good News". It is an insight to 2 Corinthians 2:15-16. — "For we are the aroma of Christ to God among those who are being saved and among those who are perishing, to one a fragrance from death to death, to the other a fragrance from life to life. Who is sufficient for these things?"

We begin our understanding with the First two Parables in mind. In reading the chapter from verse 1, we glean that there was a large crowd of sinners and Tax collectors drawing near Jesus. The Sinners and Tax Collectors represent those who rejoice over Good News, like the characters who found the Lost Sheep or a Lost Coin in the first two parables. The Good News is, of course, the Gospel. It was this type of crowd that was gathering near Jesus because they were excited to hear what Jesus had to say, the Good News. So, the first two Parables are about those who rejoice over Good News!

In verse 2, we encounter the opposition who represent the older brother or those who hear the Good News but the News has a negative or opposite effect on them. Instead of rejoicing on the news of Salvation by Grace, they begin to justify everything by their works...

"Look, these many years I have served you, and I never disobeyed your command, yet you never gave me a young goat, that I might celebrate with my friends. But when this son of yours came, who has devoured your property with prostitutes, you killed the fattened calf for him!" —Luke 15:29-30

The Older Brother, the Pharisaical Jew, who all his life observed the customs and stayed away from the unclean traitors and Gentiles. He thought to himself, "sinners! The lot of them!" "How could God, the Father, and the heavenly host have a feast over such?!"...

And the Pharisees and the scribes grumbled, saying, “This man receives sinners and eats with them.” —Luke 15:2

It was that comment that caused Jesus to tell them the Parables about the different reactions to the "Good News". And so the telling began in verse 3: "So he told them this parable...".

Why was the prodigal son spoken harshly of by his elder brother?

Three reasons come to mind.

1. When the prodigal left he took one half of everything and left his brother to do all of the work including care of the father in his old age.

2. A son asking for his inheritance is like spitting in his parents face and saying that the only value they have to him is as a source of money. It is difficult to think of a greater insult.

3. There might have been concern that the father would grant the prodigal another “half”.

The last concern was put to rest by the father;

Luke 15:31 And he said unto him, Son, thou art ever with me, and all that I have is thine.

The brother's harsh remark betrayed that he had the spirit of a hireling, rather than that of a son. His service for the father was not one motivated by love, but rather by the promise of reward. Even though he had not gone into great excesses like the younger brother, his failure to show the same concern for his lost brother as the father did showed that he had yet to discern the love of the father for himself. "He that loveth not knoweth not God, for God is love" (1 John 4:8).

Ellen White in her commentaries in the book Christ's Object Lessons goes into great detail.

By the elder son were represented the unrepenting Jews of Christ’s day, and also the Pharisees in every age, who look with contempt upon those whom they regard as publicans and sinners. Because they themselves have not gone to great excesses in vice, they are filled with self-righteousness. Christ met these cavilers on their own ground. Like the elder son in the parable, they had enjoyed special privileges from God. They claimed to be sons in God’s house, but they had the spirit of the hireling. They were working, not from love, but from hope of reward. In their eyes, God was an exacting taskmaster. They saw Christ inviting publicans and sinners to receive freely the gift of His grace—the gift which the rabbis hoped to secure only by toil and penance—and they were offended. The prodigal’s return, which filled the Father’s heart with joy, only stirred them to jealousy. In the parable the father’s remonstrance with the elder son was Heaven’s tender appeal to the Pharisees. “All that I have is thine”—not as wages, but as a gift. Like the prodigal, you can receive it only as the unmerited bestowal of the Father’s love.

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It is true that you claim to be a child of God; but if this claim be true, it is “thy brother” that was “dead, and is alive again; and was lost, and is found.” He is bound to you by the closest ties; for God recognizes him as a son. Deny your relationship to him, and you show that you are but a hireling in the household, not a child in the family of God. Though you will not join in the greeting to the lost, the joy will go on, the restored one will have his place by the Father’s side and in the Father’s work. He that is forgiven much, the same loves much. But you will be in the darkness without. For “he that loveth not knoweth not God; for God is love.” 1 John 4:8.