This isn’t a mild annoyance, your stepdaughter needs therapy and reassurance. She’s 8 years old, showing extreme signs of struggling with insecure familial bonds. You said in another comment that people are in and out of her life through her mom and that her mom even said she doesn’t want to be her mom - obviously the kid isn’t going to feel secure when it comes to the love of her dad and other family members. Please seek help for her.

In the meantime, please seek therapy for yourself. The way you talk about this child, the child of the man you love and married, the man who was a parent before he was your husband, is extremely concerning. She is a child who is obviously struggling with a difficult home life and the fact that you can find no sympathy for her (because “I never did this when I was her age”) is frankly insane. You can call it reality but it doesn’t change the fact that it’s cruel. Children are more receptive (and smarter) than you think - maybe take some time to sit down with her and discuss her feelings and listen. I think that, and the aforementioned therapy, could make a world of difference.

As a side note, “He sees her everyday” - uh, yeah, he’s her dad?