How do you tell your sister-in-law to leave you and your husband alone when in an argument? Mine is very intrusive and I need to put a stop to it. Please advise!

If it's parent in law or an elderly type of relative, I'd let it slide. But if it's a sister in law, who's around my age, I'd tell her to mind her own business. She wouldn't like it if I butted in her personal arguments, so I'd expect the same courtesy from her. But that's just me -- yes, in laws are super tricky and it's always good to pick your battles well.