Well, girl, fuck him then, because then you can get married and be a part of your best friend's life forever, because you'll now be sister-in-laws. Yeah!

Typical girl behavior.

so first of all you should ask if your sister's brother is single and then you should ask if she wouldn't have any problem with you dating him because it would not affect your friendship you can allocate time between your friendship and you're dating her brother it's like any regular person does and that if you guys break up you promise not to make it a big thing because you know it's a family thing