Help! My sister-in-law is breaking our love bond

PIC: My sister-in-law tells my wife that her over due pregancy is due to my supposed infidelity

I have been with my fiance for three years and she's now pregnant and in her late third trimester, her sister came to help her until she delivers. However, I do not like my wife's sister because of her behaviour. She calls different men all night, demanding money from them. Despite my sister-in-law being married, she tells my wife that her over due term pregnancy is as a result of my cheating. She is negatively influencing my wife's behaviour. what should I do, advise me.

Richard l