I do understand your pain. I have problems with my monster-in-law and it is not easy to go around it without causing a revolution in the family.
If all things are taken and it really isn't a medical condition, you should keep your distance and not feel guilty about it. Just do the 6 month obligatory dinner to keep the peace between all parties and move along. With a child on the way, it is important you keep a healthy environment.
What is important is that your husband seems to be supporting you. His parents might be more difficult to convince because parents are sometimes the most obstinate to accept or even consider the existance of a problem with their children.
Suggestion: Find a qualified person. Tell them what you have published here.
Tell him/her about the delicate situation.
Arrange a meal where she / he will be present to witness at first hand what is happening.
Subsequently discuss it.
Then you can go about the matter in a responsible way.
Good luck.
Whatever, your marriage will benefit, when your spouse observes your care in seeking an improvement
in the quality of lives of his blood relatives.