Unless she is still married to your dad or his dad is still married to his mom, I don't really see an issue with her having a relationship with the dad. However, if either of them are married, I can see you having an issue.

I would have a major problem if my mom was dating my boyfriend's dad. That's just something a mother shouldn't do. Move on OP and spend time with other members of your family or your new partner. I'd cut her put of my life but that's me.

Depending on how long they have been together this really isn't her being a bad person, just her choosing which group of people she loves to spend Christmas with. If it really mattered to you you could have it a day early or late. On her dating your boyfriends dad I do t see how that could be an issue for any relationship involving the four of you.

This does all depend on whether one of the parents is cheating for them to be together. If not, I don't see what the issue is.