Again, the only advice I can offer is to give the entire evening to Jesus. This is how I give something to Jesus, in a very practical way.
I get out a blank piece of paper and I spill my guts - I write down every fear, every thought, every wish, EVERYTHING about the upcoming event. I do not worry about grammar, syntax, spelling or even if it makes sense. It might go like this:
Dear Jesus, I just know that this entire evening will be a nightmare. The BIL will tell everyone I am a crazy, bitchy religious fanatic and my in-laws will tell everyone what a loser I am and how I am an unfit mother, too ugly for words, my food sucks, my children are retarded and that their son was drunk and loaded the day he married me. They will destroy my home, kidnap my children, call me horrific names and…blah blah blah…
You get the idea. Then when you are done, fold the paper in half. Get a shoe box. decorate it as your “Jesus Box”. Get on you knees. Put the paper you have written on in front of you and say:
Holy Jesus, here it is. My fears about this party. My thoughts and fears about this family. I am unable to make them do anything I want them to do, to behave in the proper way or to have any respect for me whatsoever. Therefore, Lord, I give them and my fears to you.
Then put the paper in the box, put the box on the top shelf of your bedroom closet (and not out in the livingroom so one of the inlaws can ‘accidentally’ find it), do the other prayer I listed and have your party. THEN, if you truly want to be free of these people keep them OUT OF YOUR LIFE…no matter HOW inconvenient it may be in the future. Change your telephone number and do not give it to them. If you have to, take out restraining orders and serve them with them…but do NOT welcome them into your home anymore. You cannot change them, you never will and you might as well just give up. Trying to control their behavior is making YOU nuts.
And of course ask the priest over. Though I truly doubt that, if these people are the psychos you have described, it will make a lick of difference in their behavior.
Good luck.