So, like many other common issues for step-moms, this is probably going to sound really petty and dumb. But I think it must be something other people have come across...
I will be marrying a father of 2 girls (6 and 9) in September 2014. Their mom has main custody, he has them every other weekend and two evenings a week.
Last week I noticed he does something that bugs the heck out of me. When referring to his ex in context of the girls, he calls her mom. Eg - (to his daughters) "it's time to go to mom's!" or even to me "They will be at mom's this weekend". Sometimes he says "your mom" or "their mom" instead of just "mom" and then it doesn't bother me at all.
I tried it out myself. When we were out with the girls I said "Where's dad?" (I always, always have said "your dad" until now). What I discovered is that eliminating the "your" makes it feel like a nuclear unit. Mom, dad, and kids. If people in public hear me say "Where's your dad" it does not indicate that I am "mom" or that I have any intimate connection to him. If they hear "Where's daddy" it does indicate that I am mom, and that he and I are part of a nuclear unit. So his referring to his ex as mom makes it feel like they still have this emotional and physical connection as mom and dad, and that they are a unit, and it makes me feel very consciously outside of that "mom and dad" relationship. If he says "their mom", he is acknowledging the girl's connection to her without there feeling like he is still a part of that family unit in the same way.
Does any of that make sense? It seems silly that one little word is so loaded, but in step families words like mom and dad ARE loaded! I would be interested to hear if anyone else has thought about this and how they dealt with it.