The thing about being a step-parent is... well, it's pretty much bullshit.
A step-parent holds all of the shared responsibilities as the other parent--financial, moral, as well as putting up with eye rolls and backtalk--but does not have any real parental decision-making powers. At the end of the day, if I think Ace needs to attend Preschool A over Preschool B, my opinion is only a small part of Ant's decision-making process. Being the biological parent and legal custodian, these are his decisions to make. I'm not saying this is wrong... it's just how it is.
So, although a step-parent has played an active role in potty training, spent beaucoup dollars on birthday parties and rushed a fevered toddler to the ER... there are many cases in which it is evident that you are not actually the parent, regardless of what parental roles you have taken on.
So in the midst of various situations in which you are constantly reminded that "your children" aren't really yours, you are expected to love and treat them as your own. (And you absolutely should.) Especially if you have your own biological child in the household. Because if you treat any of them differently or unfairly or in a way that could be misconstrued as unfair in any way, shape or form, not only will the children feel it and probably feel sad or awkward, but your spouse is likely to fly off the handle in defense of his/her children.
Even if your family is significantly less dysfunctional than the rest of us and you've managed to create a completely equal atmosphere for all children in the household, you still aren't their "real parent" and the kids know that. It doesn't matter that their biological mother (or egg donor) has never paid a dime to support them, supplied with with food or clothing, picked them up for visitation on a regular basis or even attended their last birthday party... she is still Mommy when she does get around to giving a shit about them and she is going to be held in a very special place in their hearts. (And she absolutely should be.)
As a very involved step-mom, I know that I too will have a special place in the boys' hearts. And when they're 22 and 23, they will likely have a better grasp of the situation and love me in the same way they would love a biological mother. But for now, at the ages of 2 and 3, Mommy is the superhero who shows up once a month to whisk them away for a weekend at her house, and Amie is their evil step-mother who makes them follow the rules again when they come home. And it would be an evil thing of me to point out otherwise. So I won't.
