When I read the OP, what came to my mind is that Martha and Jim may have severe marital problems. Jim has withdrawn and Martha is frustrated, yet unwilling to share her feelings with HER family.
It reminded me somewhat of my 2nd marriage. He, too, was great with his family -- spent time with them, helped them out with things, but when around my family, he read a book, didn't converse. Most times, he just didn't attend my family dinners, etc. I made excuses for him -- he was busy, he was tired. Ultimately, one day he came home from work and announced that he didn't want to be married anymore. He packed up and left the next day. Nope, no other woman, he just didn't want to be married any more and that was the end of that.
So, if the OP writer talks with Martha, one-on-one, and asks why she seems so rude, tense, unhappy and is there anything the parents can do to help. It might result in more information which puts behavior in context.
I think accusing her of rudeness, pointing out her neglect and disdain, etc., would make Martha defensive and drive a further wedge between the family members.
On the other hand, perhaps Martha really is just a shitheel who needs some time by herself.
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