Those women with whom nikaah is not permissible

Q: A born Muslim girl is married to a Christian boy with her fathers consent. Her father (our close relative) told us that his happiness lies in his childrens happiness.

1. Can we break our relatioship with that relative and his family.
2. Even now also he does qurbani  in his daughter name who is now a christian. Is that ok?

Q: Is it permissible for one to marry his divorced wife’s sister while his divorced wife is in iddat?

Q: Is it permissible for a person to marry two sisters at the same time?

Q: Is it permissible for a person to marry the mother of a woman he divorced before consummating the Nikah?

Q: Is it permissible for one to marry his ex-wife’s mother?

Q: Is it permissible for one to marry his ex-wife’s daughter (from her previous marriage)?