What about a sister who hates her husband?

Your husband's sister seems to be spoiled. No sense of responsibility, no responsibility for one's own life, no respect for others, no consideration for others, no filial piety to parents.

Sometimes you need to tell her by actions that you shouldn't take things that don't belong to her. If it's not her turn, you should wait. Although she is a relative, when she has invaded your life, there should be some rules. Her mind hasn't grown up at all, and she thinks she is a child. She doesn't feel that she needs someone to take care of her and think of others. She doesn't think you are a new family, and she is no longer the wayward little girl she used to be.

You and your husband need to help her realize that she is an adult. You should tell her what is unreasonable and stick to the principle. It is really bad for her to be soft-hearted and spoil her. It's time for her to learn to be an adult.