I have what may seem like an odd thing to add... My "folks" have been married for about 25? years... That is my Dad and Step-mom. I have never called her my mom... she has been around since I was 8, but it just never seemed right to me. I don't think it ever will at this point. I have a Mom... she's not a wonderful Mom that was always around... actually, I feel more like her parent most of the time... but she's my Mom. My stepmom has always felt bad that I don't call her Mom... I call her Karen, I know it hurts her feelings and yet, I hope she understands why. When I talk about my folks house, I mean their home. I would hope that stepparents as well as birth parents are very careful to follow the childrens' lead in this situation... they will do what they are comfortable with.
Just my two cents...