Most Helpful Guy
For example, I've been with my wife for nearly 6 years. If we suddenly got divorced one day (i. e., a break-up), and my brother walked up to me and asked me to "date" her, or wanted to talk to me about his "interest" in dating her, I wouldn't take it well.
I mean, it's someone I care about. It's someone I struggled through life with. It's someone I had great sex with. There is too much history and too many deep feelings and memories there. Knowing that "she" can just move on (i. e., not really the fact that my brother is dating her) is what would make it emotionally uncomfortable. Although, the thought of my brother now having sex with her would also be emotionally uncomfortable (and also corrupt those positive memories of the great sex).
Now, if I was single, and my brother broke up with his girlfriend, it would be a different story. Why? Because he never had strong feelings for her. They never struggled through life together. The sex was "meh." He's not really happy. She's not anything amazing (in his eyes). Now, I'm not saying she's not amazing objectively, or to someone else.
So, if I were to go up to him and tell him (after they broke up), and tell him that I was "interested" in maybe dating her... I honestly don't think my brother would give a sh*t (not just externally, but internally too).
Now, in the long-run, it might get awkward. It would be interesting sitting across the table from each other each Christmas knowing that my brother fucked my wife. It would be interesting having my wife go pick up the kids from my brother's house when I'm at work, and me knowing that they had both had sex in the past (and I will just have to accept it on blind faith that nothing is going on between them when I'm not there).