How to deal with new young step mother?

I'm 15 and my new stepmother is almost 20 (yes..I know my dad is crazy). My little brothers are 5 and they have started calling her mommy. My dad has asked me to call her mother as well so I won't confuse my little brothers. I don't mean to be rude or disrespectful but does he really expect me to call this woman (who is only like 4 years older than me) mom? I don't think I can do it. I want to talk to my dad and tell him I cant call her mom. Do you think my step mother will be hurt? Am I being rude?