I was in a similar situation back in November. It was a few weeks after my H moved out, and my in-laws said they were going to be in town, and wanted to go to breakfast with me. I started off doing the same thing you did - I was dreading it, I was trying to figure out what they were going to say and how they were going to act, and how I was going to respond and how I was going to react, and boy was it exhausting.
I decided (with the help of some more level headed DB friends!) to go into it with no expectations. Just open heart, open mind. So I did. And honestly, it was lovely. Probably the best time I have ever had with them.
Try to remember that your in-laws are not your H. They are not responsible for his behavior (hell, they probably don't even know about most of it!), and they are probably hurting too. Try to focus less on the fact that they haven't been in touch for a while, and focus on the fact that they reached out and want to spend time with you and the kids. It will probably feel awkward for them, too.
If I were you, I would just stay off the topic of your H completely. You don't have to fake anything - but you also don't have to tell the whole truth or vent. Think about what you would tell someone who is a casual acquaintance. When that kind of person asks how you are, you know you're supposed to say "great" even if you're not. If they start asking specific questions, you can just say that you are glad to be with them and would love to just enjoy celebrating the holiday together.
Good luck . . . I can't even imagine what you are going through right now, and I know you have been through so much already - but who knows? - maybe you can have a pleasant R with your in-laws.
me: 44 XH: 42 M 11 years D10 and S8 Bomb drop 9/27/13 D final 7/1/14