Should they be friends? I went through horrible depressing break ups at the time.

My ex's sister and I went through horrible depressing break ups at the same time. We both had children by our ex's and were with them for 10 yrs. Needless to say we were there for each other bad mouthing the new girlfriends, and trying to help each other figure out why the guys would cheat on us. So here is the question can i be mad at her for being friends with her brothers new girl. I feel like it is betrayal. Am I wrong?