I, too, have horrible in-laws.
My husband doesn't have a lot of family. His parents really use guilt to get him to do their bidding.
I know that their insanity has played a huge role in how my husband deals with problems.
I try, try, try to get my husband to understand what they are up to - cause dissension - and try to discuss ways we can handle them. It's hard. They are his parents. But I try to make him understand we need to do what is best for US!
I totally ignore them. I have nothing to do with them. It doesn't matter what I do, they always make me the bad guy. So what's the point?! I find I feel so much better.
Since you will have to be around them cause of the kids, keep your distance, be polite, but DO NOT interact with them.
Keep your distance!!
Not only will you feel better, but you will ruin their fun!
They are just waiting for you to do something that they can rip you to shreds about. Do not give them the chance, or any material to work with.
This is what I did and it worked. They slowly began to have more respect for me. The bad treatment has stopped. They are incapable of having a healthy relationship with anyone though.