Sister-in law issues, I need help 😥😡😩

Ok, so all this started last January when I cheated on my fiance with two other men 😞I know it's stupid and I regret it,I love fiance so very much and what I did was in the past,so please no rude comments. Anyways then in June my fiance and I got pregnant 😀💙 and now our son is a handsome, smart,adorable 8 month old.And ALL through my pregnancy so would call me horrible names like Bitch, Whore and Slut,and she's SO upset that my fiance and I are together, she says that I don't deserve him,etc. ....She's one crazy bitch, she lied to the whole family that my son was taken by cps. (Which wasn't true) I'm a AMAZING mom and I love my son with all my heart. She said that if we broke up she will try to take my son away from me. ....She calls my son Retarted because I have Develomel Delays, etc. ...She is just a VERY mean, heartless, horrible human being. ..She said "What do I need to do,get pregnant in order to get some fucking help"? .........Im deadly afried of her,I've had 2 nightmares of her trying to kill me 😢I'm afraid for mine and my sons life...I have a little Paranora and Anxiety. PLEASE HELP!