Threaten by sister

Hi....my fiance(hindu) was married with a muslim girl and had a child...as I was not aware of it...but he already divorced with her..I come to knw about it by his sister...after some time m agree with the relationship but I dont want that my family aware or knw abt my fiance past...bcoz they will hurt emotionally and may my parents loose there concions and health issues. ...it is our mutual concern to live together without telling anything to my family about his past ...but now my fiance's sister who already married with a muslim guy dont want me to marry his brother...and she and her husband continuously threaten us to disclose my fiance past in my family though she knw about my family emotional and parents health coditon....because of that m going under depression and taking medicines too... she is mentaly tortureing me without any reason...infact she recently told about me to my fiancr ex wife and suggest her to case his brother for cheating...in oppsite of that my fiance's ex- wife told her to stay away from her...as her planned failed on that now she continuously blackmailing me and my fiance for my family...and forceing us to not marry...as she is leaving in her own house after marriage to avail property and she assume that she will not get anything if his brother got married an settle his life...she and her husband continuously threating us..her husband also physically hurt my fiance and always call him threaten him dat he will kill him...please help and suggest what we can do legally..