Hi Joli,
Sorry you are going through all of this stress. I also do not get along really well with my sister in law or mother in law, as we have had some confrontations in the past (although we all are on cordial terms at the time). The minute my husband starts doing things with them without me would be a dealbreaker for me for the relationship. I believe that if there is a problem between one of us, then this conflict needs to be explored and addressed. The minute people stop communicating and begin not inviting people over to places is unhealthy. If my husband's family cannot be nice to me and think that they can deal with this by hanging out individually with their son, well then this would make it that much easier for me to end the relationship (but this would only happen if my husband permitted them to do this, which so far he has never done. My husband is against doing things without me).