How do I stop this ??

So me and husband been married for 3 years and 3 months. We have a lovely 7 m/o lil baby boy. We are living a happy life as far as it seems. But from recent past experiences as let’s go back before Marriage , we were engaged for 4 years and through out that period, we were falling apart ( due to long distance relationship) but we made it thru and we are so good. So how it’s going to verge of distraction *interference of Mother in Law and sisters in laws*. His sister lives with us but his parents moved back to their homeland. Sis in law is a divorceè and currently she’s spreading false news about us two. She manipulated her parents telling them lies and proving herself innocent. And not only that she goes out with her friends and we know nothing bout what she’s been doing and where she spends her money but that wasn’t the problem till now that I’m going to visit my parents soon n she told her mother ( who’s living in same place as my parents) that her dress is missing and she never received one she sent. She asked me on phone and never ever MIL sent me anything ( she does to her older daughter in law and her daughters but not me) . So I said I don’t know anything and then SIL said it’s cool no worries but now I found out that this is the 10th time in this month and she’s accusing me for STEALING . My husband and I went into huge fight becz of her and it’s on going that it’s effecting our relationship so badly. We can’t ask her to move neither we can (due to religious customs ) .. so now my hubby and I aren’t talking to each other and it’s been on going like every other week or 2wice a week...