J.F.
I agree with all the other advice. Talk to the dad now. Don't wait. If you wait, situation might get worse.
I know that being on the other end from you, I am a step mom, you need to talk about it and try and resolve the issue as soon as possible.
My husband has 3 children from a previous marriage, and I have 1 from a previous relationship. We have two together, so we have 6 between the two of us. When my husband and I got together, i had to deal with his ex-wife ( no offense to anyone) but she was a real piece of work. She would call to our house ask to talk to my husband (always seemed like when he was at work) and then try and cuss me out. I finally told her, call he cell.
After about 2 m onths, she started acting nicer. Then about 6 months after my hubby and i were together, she was ok. She even called me up one day and thanked me for taking such great care of her daughter while her daddy was at work.
So i know that step parents and parents can get along.
Good luck! Hope all works out for you and your kids.