One year when my two adult girls were in a feud, I told them unless they resolved the matter, they could not be at my house at the same time. The oldest could come over the day before Christmas the youngest could come the day after Christmas. Neither could come Christmas Day as no one wanted to be around negativity or be reminded of why the other wasn't there. DH backed me up with no exceptions to our rule. By Christmas Day they had made up and acted nicer then when they were at odds.