Congrats on your little bundle of joy. Being a step mum is never easy, and I don't know that there are any right words to say in your situation.
In some ways it is not a bad thing if you and the girls have made the decision not to deal with each other, and as for your DH not being supportive, I have to say I have found that to be the case with my partner when it comes to his boys. We have been together 4 years, and whenever there is a conflict surrounding his kids, he takes their side.
I don't know that it will ever change, it is something about them and their kids.
But that said, don't feel like you are alone out there. Us Huggies Step parents are here for you. We cannot solve your problems, but can be here to chat.
You never know what might happen, but maybe once your little man is here, the older girls might have a change of heart. It is very hard to ignore a beautiful little bub, and especially one that is your brother.
Take care and keep your chin up. Anytime you want to chat, we are here.
All the best....