I was never a step parent, but I am sure it is a very difficult thing.
You are not alone in the fact that your husband doesn't help. I know plenty of women who's husbands don't help with the kids, especially if the wife doesn't work outside the home. I think if I were you, I would just stop helping him take care of his kids so much. I know you probably feel obligated, but if he is going to take advantage of you and not chip in, then he is wrong. If the kids are old enough to talk, when they want or need something, have them ask their dad. Tell your husband that you are not there to be the daycare provider, or the maid. I know this sounds mean, but if you don't speak your mind now, you will end up with resentment towards the kids and him. Resentment is a difficult thing to get rid of.
Hope this helps.