I'm a little confused as to why you're upset because they WILL be half brother and sister whether you like it or not.

No you should not be upset. If it's her father's baby it's her brother. This isn't about you so try to get over yourself.

No you should not be upset. My daughter calls her father girlfriend kids her sister and it doesn't bother me at all and they are not married. They are children, don't take that away from her she probably enjoys having a brother

Whether you like it or not he is their brother.

You shouldn't be upset because that's what that little boy is. He IS their brother, he is their half brother. Your ex and this other woman might not have been together long, might not be married, but that doesn't matter. The child he is having is the sibling of your girls. 

Ummmmmm you do understand that IS her brother right?

Who cares lady?!?! You shouldn't!

Idk why ur upset if he tells ur daughter shes going to have a baby brother yes lady news flash thats her brother weather u like it or not get over it ur not a little kid no more

No...seems like you are the one who has issues. Don't matter if they are married or not...it's still your daughters half brother Get over yourself.

It sounds like you're jelous. Idk what the issue between him and you are but regardless of the situation you should stop saying "her baby" it their baby. It doesn't matter if they are married and only been together 1 year if you daughter father is having another child with another woman that will automatically become her half sibling because they share the same parent! Learn to deal with it even if you can't accept it...