M.K.
Oh boy, what a dilemma!! First of all, you setting down any rules while those kids' parents are right there is going to be like talking to a wall! I had a similar situation with my kids and their cousins; I just told my kids not to play with them and stick by me. Then when their parents asked why they didn't want to play, I simply told them because their kids are too violent. I found things to do with my kids - taking a walk or playing - it was a win-win situation for me; I got away from the in-laws and my husband could visit with his family.
As for going at all - you don't say anything about your family but it certainly doesn't sound fair that you'll be spending three in a row with your husband's family. Is your family planning anything? I would discuss the options with your husband. Maybe if he is adamant about going, you could rent a hotel room instead of staying with everyone else? For me, that would cut down on some of the stress!! If you do end up going, I would just look at it as a free vacation and make the best of a bad situation!!
Good luck!!