Is it bad to date your friends sister.?

it seems really wrong. and to be honest he really smothers her. like it seems like he doesnt let her have friends that arent also friends with him. i noticed that she wont hang out with us at lunch and i think its to get away from her brother. she is really a great person. there are some things about her that could use some work but for the most part she is really good. its just that her brother, always wants to be around her and its really stifling. like even if she said yes to me, how would we even date.