It's simple. Call of the wedding.
I can't be getting married to someone who is having an affair with my brother.
Although, it will be difficult for me. I will feel hurt, used and played.
I don't know what I will ever do to my brother and the lady. But she should never come close to me for the rest of her life.
Having sex with my brother just a day to our wedding? Damn. She must have been doing that previously and may even continue doing that even in our marriage, so why continue with her?
I'll simple call it off and remain single for a very long while because trusting another lady might be really difficult.