What you wrote weirds me out and raises some red flags. I could say he's just a concerned and loving guy, but you said it's only directed at his stepmom and not his dad. How old was he when she became his stepmom? Yesterday I read a similar situation in a Dear Prudie column on Slate. Here's the link and I will paste the question and response as well:
http://www.slate.com/articles/life/dear_prudence/2012/10/...
Not My MIL: My husband's first wife died and he maintains a close relationship with her family. He is particularly close to her mother, whom he calls "mom" (his own mother passed away at a young age). Throughout our marriage I respected their relationship, although there were times when I wondered if he was doing a little too much. For example, he has given her a very large sum of money when she bought a new house, or when her nephew got married, etc.—more than what we could afford. Another time, her daughter was meant to visit her but had to cancel the trip last minute—so he invited her to come with us on what was meant to be a romantic getaway so she wouldn't be alone on her birthday. She had surgery a couple of times and both times my husband took unpaid sick leave to take care of her. She's been having medical problems but does not want to live in a nursing home. Her daughter lives in another country so my husband now wants her to move in with us. I could see myself living with his father if need be, but this woman has minimal ties with me. We've had horrendous arguments over whether to live with her or not. He says she is his mother and I should not be so heartless. I certainly don't think of her as my MIL and I don't want her to move in with us. What should we do?
A: Even if his former mother-in-law was his actual mother, I would object to the place she has in his life. People should not go broke to support their parents; they should not ruin romantic weekends with their spouses to accomodate their parents (except for an emergency); and unless both spouses are in favor, they should not move their elderly parents in with them. The problem here is not convincing your husband this woman isn't really his mother, it's that he's undermining his marriage by placing her needs above yours. I try not to end every letter with a call for therapy, but here goes. Stop having the horrendous fights and get the two of you to a neutral party to help you negotiate how your husband can feel he is honoring this woman while respecting your limits.