The main problem here is the husband. If he were mine, I would tell him straight out that he is being a bad husband and father by allowing, supporting, and defending his sister’s treatment of his wife and kids.
He is also violating the Biblical statement that a man leaves his family and cleaves to his wife. She is his number one priority, not his family of origin. So if he considers himself and upright Christian husband and father, he needs a big whack upside the head about how he is not behaving that way.
However, MaggieC, I am sure you are not shocked. You probably knew he was wimpy when you married him. So, since you made the choice to accept that in him, I’m not sure it’s now fair to call him out on it. Perhaps you thought he would outgrow it, or that surely his wife and children would be the one thing he would fight for and defend…
Girls, make sure your guy proves you are number one BEFORE you marry him. It will save you a lot of heartache in the end. My DH had to draw some boundaries for his mother, but he DID IT. He told her in no uncertain terms that if she forced him to choose, he would choose me. Amazing how that makes even the most dedicated meddlers back off!