Good step-parents take on other people's children and it's not easy. They face opposition from the child and the other parent at times, they take on a financial burdens of that child, and they also show a special kind of love for a child they have no biological bond with.
Step-parents step into a role of someone who makes a covenant to a person and their already made family situation. They already know that there is the possibility they will hear "you're not my mom/dad" statement, or be plain disrespected.
No matter how many children are involved, step-parents give willingly to the household which helps nurture the child; and that's done with love.
They are showing a love that isn't "step" but the love of a parent to a child; even it he/she isn't theirs.
That should be recognized.