I've never been directly affected by a divorce in my family before, however I have relatives who have been thru it. My aunt (my mom's younger sister) divorced her first husband. They were married too young and did not get along. She doesn't really talk about it because it was so long ago and she is happily married now with a great guy.
My aunt (my dad's little sis) divorced her husband of about a dozen years (give or take) about 8 or so year ago. They butted heads, each person wanted things done his or her own way and they spent so much time apart with their jobs. What it boils/ boiled down to was compromise and communication.
Without compromise and communication (maybe more things I have not learned myself yet), it's a challenge to make any relationship work.
I think you're entitled to your feelings. After all, a split such as theirs could also potentially be a split between your sis and law and yourself. Not unless of course it is ok to remain in contact with her. You still consider her family, a friend, so I'd hope that it wouldn't hurt your bro if you wanted to remain close to her. I'd hope that he could understand that his relationship with her is separate from your relationship with her AND hopefully he could respect your feelings and the feelings you have for her still.
Either way, you still have your bro and potentially your sis in law as well in your life. You can still maintain relationships with each one. It is your life to lead as you see fit after all! I hope I've said something that helps. HUGS!!!