now THAT'S an interesting situation ^^
to the thread starter:
I've been in a similar situation. I dated my best friend's brother. We had liked each other for 2 years, but didn't act upon it because of how close the family was. We gave in to temptation however, and although it didn't last very long, there was no problem.
I'm still best friends with the girl. I'm always over at her house. If her brother (my ex) is there, we'll be civil..."hi, how r u" etc, but that's about it. We had a bad break up too, but I never put my best friend in a weird situation.
My best friend was ecstatic that I was dating her brother, coz at least she knew that I was a great girl and she trusted me. She still wants us together, but he's a nong.
Anyway, that being said, it can work out. If ur bestie hurts ur brother, then it will be hard. They could end up living happily every after, and your sister in law will be your best friend!
Just make sure that she doesn't ignore you, and evenly spreads her time out between her boyf (ur bro) and her friends (including urself)