So I'm in middle school and I like this girl. But she already has a boyfriend. I kind of like her sister too. But I'm too nervous to ask her out. What should I do?

Walk away.
There is a time and a place for everything and it's called COLLEGE..

Become Bestfriends with her and also become friends with her BF and then tell her (not him) u know who likes her and if she asks don't tell her let her be guessing and when you are best friends with the guy he will trust u. u will figure a way to work into it.