Dealing with difficult people?

My family's difficult person is a daughter i n law who has made my son miserable for over a year threatening to divorce him and keeping the children from him. They are still married, but just barely. She wants to come to our Christmas gathering and I said yes because she is part of the family. She is trying to make amends, but I think it might take well over 100 years!! I am set to treat her amicably, but my other children are wanting to be mean. These are all adults and I do not know what they will do. Any suggestions on handling this?