Shouldn't your best friend (30+ years) tell you if she is "interested" in your despicable brother?

My best friend "announced" to me she was in love with my brother three months after my brother "announced" he and my sister-in-law of 42 years had signed separation papers. My best friend had seen and heard my brother being verbally abusive to me, and knew that we were not on speaking terms. She even sat and listened when I told her my despicable brother was divorcing his wife because she (his wife) had retired from work and life and he was bored, and she had health issues also. I would gladly have stepped out of this "threesome" she was having with us (me and my brother) if I had just been told she was romantically interested in him. Two's company and three's a big crowd.