score:4
if [my sister] sponsors my trip, would be enough to show her bank statements?
No. You have to show your bank statements. If you do not have a bank account, you should include a statement to that effect. Even if you have little or no money in your account, and you plan to spend nothing at all of your own funds during the trip, you should show your bank statements. This is because the visa officer needs to get an accurate picture of your own circumstances to judge the likelihood that you will work in the UK, stay there longer than you claim, or otherwise violate the conditions of your admission. For example, since you are not allowed to work in Spain, the officer will want to see bank statements where any deposits can be shown to have a source other than employment.
Since you say your boyfriend will take care of your expenses, you also need to show his bank statements. You should also show that you have permission from his landlord (or his parents, or whoever owns the place where he lives) to stay with him.
There are many stories of young adults being denied visas in similar circumstances. It is a time when people are flexible, and many people of that age seek to move to new places and expand their horizons. You will therefore want to make it clear that you will be returning to your studies in Spain.