Baby momma drama

So I'm pregnant with my first and my husband has a 12 yr old. The mother is unstable and so instead of half and half time we have him pretty much full time. She don't have space for him where she is staying with her new baby and the new dad.  I take him to school every morning and we pick him up after school 3 of 5 days feed him every night and morning.  Here is my issue... My husband is still paying child support to her every month for what?? To take care of her other son??! Also she give a him all the school and sport and medical bills which he has some medical issues so it's pretty pricey at times. We have a new baby coming and we don't have a lot of money and I'm stressed out about my job and working before and after the baby. Do I have a reason to be frustrated? Does thinking this way make me selfish? I'm starting to really Dislike her and I've noticed is making me grumpy with him :/  my husband chooses to pick his battles with her but this is costing us money that I could have used on things Iam stressing about.  The court seriously screwed my husband over a while back I think he's scared of the courts taking more if he takes her.