oh steph, that had to be so rough im glad you could chat with the ex about it tho, it is amazing when 2 parents can get along even if its only for the kids sake! it really helps.
here is some insight from the other side i myself am a Smom, Smother, Smadre, aka bonus mom, step-mom, oops-i-slipped-and-called-you-mom and while dougs kids call me all of the above, they know in their heart who is their mommy, and who is their Abbie, if that makes sense! another thing that happens is doug will accidentally call me Nicole! oops, now that one is usually when he is mad or frustrated at me haha
but seriously, the kids will call me abbie almost always, unless they want something, then its the silly version of mom, madre, mother, etc until i cave. when they call me these forms of "mom" i know they dont mean "hey you, im confused as to who loves me more" they mean "hey you, im so glad you love me as much as my mom does so sometimes the word i call you sounds the same". hope that rambling makes sense!
dont stress any more over it, you know your kids unconditionally love you and always will, while their step-mom will ALWAYS have to fight for their love, thats just how it is
HUGS!!