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 you're in good terms with your SIL, you can tell her yourself. However, letting the husband handle is the best route. Ask your husband to advise his sister on this matter. Also, getting a room to discuss issues that concern a couple is an absolute must. Washing your dirty linen in public only attracts onlookers.