Sister and Parents

My sister and my husband are having challenges with each other. She is a realtor and he is a loan officer. They are working on a deal together. There was an error in the process on my husband’s side of the deal. Not his mistake, the underwriter’s. He found a way to still close the deal. Sister yells at him, insults him and goes to our parents to try to mold their opinion of husband. Mother says there are two sides to every story and that sister doesn’t lie. I don’t know what words sister said to mother. Mother has formed opinions about husband based on sister’s words. Mother has not spoken to husband about the issue. I am trying to decide if I want to request that mother speak to husband about the incident if she is going to talk about him with sister. I’m trying to decide why I want this and how I want to think and feel if mother declines request. Sister’s behavior is a pattern and I’m also trying to wrap my head around how I want our relationship to look. Sister is the kind of person I would not interact with if she was not family.