Boyfriends cousin (rant)

I've been with my boyfriend for about 2 years and at first when he was living with his cousin his cousin was really nice to me, always told my boyfriend to take care of me because he wasn't gonna find another girl like me etc. and now ever since I got pregnant she's been telling him lies, she tells him that I call her and tell her that I'm with someone else , she tells him and his mother that I'm a whore and if that's not bad enough she even told him that my baby isn't his and that I told her it was his , (it is his & everything she says isnt true at all I don't even talk to her) .... I know she's been trying to get pregnant and I wish her the best on that but it isn't fair that she's doing that to me I've never done anything to her , I feel like she likes my boyfriend ( yes even though their cousins) that and she's jealous even though she shouldn't be ..her time will come to have another baby... did i mention I'm 20 & she's 38...