Yes - dd1 has a stepmum - she calls her by her first name (well an abbreviated version).
I would be furious if she tried to make dd call her any version of "mum". Mind you, I can't stand the woman anyway...
Having said all that, dd1 calls my dh "Daddy". Used to call him by his first name but since dd2 came along, he has been "Daddy" to her as well. She's closer to him than to her real dad though and has lived with him for more than half her life.