Are buddhists also called brothers and sisters?

Usually when speaking about someone who is following the buddha teach is called a monk.

This is not true. There are lay followers and ordinated followers and the latter are the monks through both follow Buddhism.

But are monks or other buddhists sometimes also called brothers and sisters?

Brothers is a Christian concept which is not found in Buddhism.

Is it common or very rare?

This is non existent.

And what is the reason behind it?

This is like asking why an apple does not have a peelable skin. There are 2 different institutions which developed independent of each other. So there is no sharing of such concepts.

I guess it's a cultural phenomenon i.e. it varies from one culture to another.

Other answers here (from a Theravada background) say it's "never" used; but you can see it being used on the Plum Village web site, for example,

This Spring a joyful delegation of Dutch-speaking monastics from Plum Village and the EIAB (including Brother Phap Ung, Brother Phap Xa, Sister Jina, Sister Lan Nghiem, Sister Sang Nghiem and Brother Pham Hanh) will be leading a series of mindfulness days and retreats in the Netherlands.

I don't know whether that example is a peculiar adaptation to the conventions of English and French language and culture, or, whether the "brother" and "sister" might also be used in Vietnamese (I think Plum Village is Vietnamese Zen in origin).

Depending on the culture, "familiar" terms of address might be widely used in non-religious social contexts too -- e.g. apparently it's common or conventional in India to address women as "auntie" and "sister" (see e.g. Auntie mean when Indians say it?).

And what is the reason behind it?

I went looking for the origin of the use of "brother" in the Christian tradition, and found this answer:

The word 'brother' in Hebrew does not mean 'the brother' in common understanding. It could also refer to cousins and other relatives. The similar understanding of "brother" and "sister" is in antient Greek and Aramaic.

So I guess the reason for calling people brother and sister in Christianity are:

• Cultural and linguistic, social
• Traditional
• Originating in the words of the founders of the church (for example Matthew 12:49)

They (using "brother and sister" in Christianity and other religions, and not using "brother and sister" in Buddhism) might be two sides of the same coin? Christianity might encourage you to address non-relatives as if they were your family -- perhaps to encourage generosity and a universal (non-exclusive) love.

Conversely it seems to me that the origins of Buddhism encourage people to not treat your own family as special (my saying this is an exaggeration but perhaps you understand what I'm saying).

Buddhism also encourages universal relationships (e.g. "sympathy towards all living beings"), although that may be slightly more of a Mahayana concept than Theravada.

It may be that there isn't a "reason" i.e. the reason is simply an accident of history and language; but if I had to create a reason I might guess/suggest it's because Buddhism arrives at a universality by denying or de-emphasizing (keeping detached from) special relationships.

As a footnote, I just found on Wikipedia that, in Chinese, a Buddhist teacher might be addressed using a term that's used for "master" and "father" (i.e. using a word that might be translated as "father"):

Sifu - Common usage:

Likewise, since religious instruction involves a teacher-student relationship akin to apprenticeship, Buddhist monks and Taoist priests are also addressed as sifu or shifu.

I don't know but perhaps that implies that (in Chinese) you might address your fellow-students as brother and sister.

Male and female students who began training before you and are thus senior, are Shixing "teacher older brothers" and 師姑 shi gu "teacher's sisters". ... Students junior to you are your Shidi and Shimei.

No, when speaking about a monk, you should use respectful words like 'Venerable', 'Bhante', 'Reverend' etc. If you are speaking of a layman, it's ok to call 'brother' or 'sister' or any other decent way of addressing.

I know you ask about contemporary Buddhists, but for what it's worth, Buddha often called young women "young sister/little sister" when he gave them Dhamma talk. I think it's cultural and linguistic (just like what Chris said).

Examples: when Cincamanavika accused Buddha to be a father of a child in her womb. Buddha said to her ""Little sister, only you and I know whether you are speaking the truth or not" or when Buddha told Lady Patachara that he could help her with her griefs. "Little sister, you have come to someone who can help you..."

Its a sign of familiarity and compassion.