Heartbroken, when we had our first gathering with my family after DDay (Christmas dinner) I knew my mother would be a problem. In her typical conflict avoidant way, she wouldn't tell me that, but she told my sisters that she didn't know if she would come since my FWH would be there.
I made some time to go and talk to her in advance, and with some coaching from my IC I managed to make her acknowledge her discomfort and then I asked for her support and told her what that meant to me. She came, and she did great. She even hugged my FWH when she left and told him she was really angry at him, but that she loved him. I almost fell over.
Has your XH had a talk with his family? I think that's a good idea because even if you think you have your big-girl panties on and are prepared for it, it will be hard on him if his family makes it difficult. I think a bit of a united front is called for in this situation.
Edited: You posted while I was, so I see he already talked to them. Since that's the case, I just hope our experience with my family gives you some cause for optimism that it will be a good visit.