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!

In-laws are pretty tricky to handle. For me personally, if it was anything in-law related, I would get my partner to settle it instead because he's in a better position than I am and also would be able to better understand them so his approach would seem less harsh as compared if I do.