Get a card, print out the above letter and smile. I believe you sister in law would benefit from reading what you wrote about her and seeing how you feel, uncensored!
Everybody needs an ally--sounds like she is yours.

Invite her back into your life and enjoy her as a friend. She obviously has character
since she still acts to you as before regardless of your brother. Let her worry about that, you enjoy her friendship

There, I will now get down off my soap box