text: i'm in love with my best friend's sister, what do i do? i'm starting to fall in love with my best friend's sister, she's two years younger than me. i'm not sure what to do talk to your best friend about it.  and you should be very respectable about it to.  don't just go to him and say "hey i wanna bone your sister really bad".  all joking aside, if you guys are close and your a good guy, he will be glad she is with you and not some scum bag.