Salty Baby Mama's. That Need to Get Over It !!!!!!

Ok so my bf has 2 kids from a previous relationship. His kids know that I'm pregnant & they're excited. So we recently found out that we're having a girl. So I asked him when he was gonna tell them that they have a baby sis on the way. He said he's not gonna tell them anytime soon. I asked why ? He's scared they will tell their mother & she'll do something petty like try to put him on child support. My bf is a great dad & is always there for his kids. She has a bf that she lives with and they've been together for 4 years !!!!!! I've met her before & we've always been cordial. But he just fears that when she finds out about my pregnancy that she will make his life a living hell. Anybody in a similar situation ?