If I were you, I would attend the reunion dinner only if the mistress isn't present. Else I'll send my daughter there and fetch her home after they are done. As for visiting, likewise. I will put my feelings aside and focus it on my child. Your daughter will enjoy though you may not. But for that few days of sacrifice, I would still go. It is a long journey down this road, avoid now but you can't keep your child away from the other side of the family forever. When she is older, she can make her own decision.