Actually, KNOWING that one of the sides is outright rude and will most likely continue to make things difficult, would make it easier on the person who belongs to that family to talk to them and ask them to please behave. They SHOULDN’T live with either family in a situation like that.
Also, it would be worthwhile looking into why the families don’t like each other/the person. There may be something there and it’s best for the couple to know that so that they can make more intelligent decisions.