First of all, are your children adults that can work on their own relationships with their father?
Also, how is the older son a jerk? He was invited with his wife, who has become good friends with Dad's wife, to a Thanksgiving dinner. And you think it is good manners for a guest to invite several other people to somebody else's Thanksgiving celebration? Just because the host is the father and step-mother, it does not make the children any less of guests in their home. You have no idea if the son suggested that perhaps the siblings should be invited. Should the oldest son destroy his good relationship with his father just because his siblings do not have a good relationship with him? It sounds like the oldest son is making an effort at a relationship. We don't know if the other children are making an equal effort.
You might want to explore whether you are projecting a bit before definitively determining that strangers who have a relationship with an ex are jerks.
edited to add a question mark. My grammar teacher would be horrified.
