I need advice

I feel like my sister in law influences my boyfriend a lot and I’m scared one day she goes against me and starts putting things in his head. I always tell him something and then he goes and talks to his sister and she’d say something different and he goes by what she says. I have to say my boyfriend has a son with his ex and my sister in law doesn’t really get along with her but his ex is trying hard to get into his family again and I’m just scared. Mind you, I’m also pregnant.