Not being a real estate attorney, I can't directly answer the question, but I can tell you that a warranty deed is used to convey interest, so yes, if he's being asked to execute a warranty deed, that means he somehow has ownership in it, and by exeucting, he is giving up that ownership permanently. And there is nothing that will prevent her from changing her will, and I doubt there would be legal recourse for your husband to pursue.
I wouldn't sign it. I would instead ask for a copy of the will, and I would also ask for a copy of the current deed to the house, so you can get a better understanding of what is going on.
And you shouldn't feel selfish - it sounds a little fishy. If it was on the up and up, the stepmom would have contacted personally and explained the situation (whatever that may be). Perhaps your husband's father had a conditional will that left half the property to his children, with the condition that the will wouldn't be executed until her death or upon surrender of property rights. My grandparents had this provision in their wills, as they were both previously married and wished to leave their estates to their respective children. It worked just fine.
If they do indeed have ownership in the house, when it is sold, they will get their percentage out of it and can then decide what to do with the money and if so inclined can give it to their stepmom. But it's not customary to simply sign rights away.
I'm very sorry for your loss - and good luck with the current situation - S.