I have seen this sort of thing play out, which is why I find it so believable. There's so much emphasis on church attendance, I wonder how much of this plays a role in the LW's opinions. The sister is not active, maybe doesn't believe at all, and hardly attends and is considered to suddenly be a bad person, influenced by the adversary, etc. This could be part of what prompted husband to almost call the police when the sister was upset at the house. She's "dangerous." I picture these ladies are so involved in their church and church activities that they really have no outside interests and things to talk about besides what's going on at church and other church members. This makes it very hard to find some common topics of interest, that is, if the sister was even interested at all in her sister, which she is not. Of course the sister sounds like she is being blamed for her husband straying. Working moms are not thought very highly of either. This is all speculation, of course, and there probably is some exaggeration in this letter, but I think we've all heard of shunning.
The sister could also just be a miserable person to bring along on these trips, but none of that was really discussed - "she complains the whole time." "She pouts when she can't afford things and no one gets to do anything they want because she never has enough money," "She can't eat anything, so we have to cater to her diet," etc. That would be a completely different story, and I'd bet Amy would have had a much different answer.