It got to a point where they were actually hostile and rude, accusing me of not caring about their family, though I am always friendly and supportive. No one calls to see how we are doing. When we go out with his family, no one takes any interest in me, and I'm tired of trying to win approval. Invitations to visit our house are turned down politely, but hastily. The women are the social arbiters, I'll bet, so get to know a couple of them.
People often behave badly in groups but warm up when culled from the herd.
You be the host and kill them with kindness. It'll only cause grief and become a major issue and lead to dissension. Speak well of his family to him, even if you have to search hard for things to say. This is the ace in the hole. If they don't care enough to make nice with you then, they're not worth worrying about.