OK, so I'm not sure if I prefer 33girl's, aggieAXO's, or CutiePie's suggestion... y'all are mean
I'm so sorry that you have to deal with the in-laws from hell, Tracy. Mine are no picnic either... I've found that the best way of dealing with my mother-in-law when she's in one of her moods, is not to. Unfortunately this isn't always possible... but fortunately, the stick up her you-know-where only manifests itself from time to time.
I like JAM's suggestion of having your husband talk to his parents and put it on those terms. If it comes from you, then you're the bad guy. I also second the suggestion that you not open the door to a family conflict, by not going to the beach house.
Take care.