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 angry mode, i will just turn and look at her and ask her to shut up . its non of her business. sometimes directly tell them off like this stops all intrusion !

Express your concern to your husband privately and see what he suggest. It's probably safer to let him handle it since it's his sister.