As a member of a complex family, I feel it's okay to use shorthand for relationship descriptors (parent vs. stepparent, etc.) if it's okay with those involved. Also think it's okay to change language depending on context. I refer to my dad and stepmom as my "parents" oftentimes because that feels nice and simple. But, of course, if I'm discussing something that involves all of the parental units -- dad, stepmom, mom -- I'll be more specific. My stepmom often refers to me as her "daughter," and some folks take that literally. I don't mind.
My stepmom, ages ago, felt uncomfortable being called a "stepmom" -- sounded like an evil character from a Disney film, I guess. She preferred that we talk of her as "dad's wife." That was cumbersome, I felt. Now she doesn't care anymore, and I have told her I like stepmom because it describes a direct relationship to her, not just a relationship via my dad. So sometimes I refer to her as one of my parents, sometimes as my stepmom, sometimes just Betty. We're chilled out about it.
Our daughter is ours via adoption. I am not militant about folks calling her birthmom "The Birthmom" (instead of her "mother"), depending on the context. I am happy to share the title of mom with her wonderful b'mom. I think I have just come to see familial roles as complex and overlapping, and our standard language for them is often too simple to capture the subleties of all family systems. So I use terms loosely.
My sister's stepdaughter, who became her step daughter at age 5, calls my sister "steppy." My sister thinks it's cute, and was a good alternative to "mommy" or first-name-basis. Plus it kind of reclaims "step" in a positive way. Of course, "steppy" would be a hard sell for a teenager!
Do what feels right for you and your kids. Have a talk about it. Let them know that the available terms don't really do your relationship justice, and no matter what terms you use, you love them for the unique and wonderful individuals that they are.