jealous cousins?

So mostly everyone in my other halfs family loves me...with the exception of a couple of his cousins.

One in particular is younger, around 20ish. Shes got no respect ...as in home training. Purposely ignores me but here's the catch... she ADORES my man (her cousin). So ever since I started coming around she doesnt talk to me, say hi, or bye unless I initiate it. And even then she kind of snuffs away. My man even asks her why she does that and her answer is " you know I dont say hi to anybody ".. like I'm and invisible nobody. Its rude in my opinion. But I guess it's because she wasnt brought up properly. Anyway.. she's pregnant, and single...so the family huddles her and babies her. Like she does, the family ADORES my boyfriend hes a favorite amongst everyone. He doesnt have any children ...until now... I'm pregnant..we told his family on Halloween. She had the biggest attitude. everyone was ecstatic...she was off in the background somewhere ignoring everything...after the dust settled.. he walked up to her and was like why are you over here punk arent you glad your baby will have a cousin her age?? (playfully) . I got the vibe that we kind of intruded on the one thing that made her special and the fact that it was me didnt sit well with her. I've no idea why she is like this. She uttered an "I guess " ... I went to sit somewhere I didnt want her attitude to ruin this moment for us. I saw him lean in to talk to her . after a minute he came and hugged. me. After we came home, he apologized for how shes acted around me for a while, he doesn't understand why shes likes that. He told her that if she didn't stop being ugly towards me then she can not talk to him as well that im family now and she has no reason to be immature. not to ask him for shit or even look our fucking way .. that made me feel so good that he finally said something about it.

I've done nothing to the couple of cousins to make them act like this towards me. I'm polite and cordial with everyone. Younger me would have called them out, lol but my love has it in control