What to address a step parent as?

My dad has a fiance that I know little about because she spends all her time around him and I don't feel comfortable with her. When I am speaking about her with my dad I call her by her first name-Shelly-but my dad basically told me I HAVE to call her Ms. Shelly. I have already had a step mom previously to which I referred to as mom because she has been there since before I could talk. During my time with her I was aware that she wasn't my biological mom but I considered her as such. I haven't seen her for 3 years because her and my dad separated. He just recently got his divorce finalized. I have only spoken to her a few times the first 2 years but haven't since because she has seemingly moved on with her life and has a baby. I have accepted that those years of my life were in the past but I do not believe that I should I have to call my dads new fiance Ms because I'm not comfortable with it and its too formal seeing as that I am 15, I think I am well aware of the implications of blended families(she has 2 sons-one 14 and the other 9) and I feel that the only way I can be ok coming to her for anything is if I can call her by her name. What to you think I should do and should I tell my dad even though he seems to be only concerned with her comfort...