Salty Stepsister

A few months ago when my stepdad was talking to my (27 year old) stepsister about how she would feel if she had another sibling, she said that it’s not his turn to have kids it’s hers. Mind you she’s not married, doesn’t have a steady job, and doesn’t really know what it takes to be a responsible parent, so I don’t even know why the fuck she said that. Well recently my mom actually got pregnant. My stepdad was holding it off because I think that he was nervous of the reaction that he would get from my stepsister. Today’s Christmas and my stepdad finally told my stepsister. She got mad and said “ I just don’t know what to say.” when she came to my grandparents she didn’t speak to anybody and barely spoke to my mom. What’s her problem? She’s 27 years old she shouldn’t be jealous like she’s some 8 year old. I’m fifteen and even I know that’s wrong. I’m excited to have a new sibling because it’s something that I’ve always wanted. She shouldn’t be upset that my stepdad has someone else to love that’s not her, she should be happy that a new life is coming into this world but she’s just bitter about it. What’s her problem? Should my mom say something?