Baby mama drama

So yesterday my fiancé baby mama brought his kids over and they talked about custody she says she is gonna take him to court for full custody and he asked why when I want to have them and she says because I know you want them so I'm going to do everything I can so you can't have them. What kinda a person does that idk what's wrong with mothers that have kids and only think about there selves why not think about what is better for your kids it drives me insane that some mothers can be such self absorbed children and will do whatever they can to hurt the other person.