A tale of two reactions
This story might illustrate how bio and step parents see and react to things.
My son is getting ready to graduate shortly. My SD, who I've always felt didn't know her role and thinks she's "little mommy", is putting together a "care package" for him for college. Things like pens, toiletries, etc.
Innocent, right? But it angers me. These are the things that I want to get for my son...that parents do for their kids...He's my only child...I'd like to take care of him...
My wife's reaction was "oh sweetie, how lovely. How thoughtful." But my reaction was one of anger and resentment. "Tell her to butt out. We'll take care of our son."
Is this irrational? On the one hand, I don't think I'd have a problem with it if it were my bio daughter. Or even someone else, like my Mom or my sister-in-law. So it is definitely personal about her.
But, why about her? It's not a hate thing, even though I have to admit I don't like her. The reason is because I do believe she doesn't know her own role, and so I'm threatened by such things.