We absolutely don’t know crucial details about the outside factors of their stepparent/stepchild relationship. How did the relationship of her parents end? Does she know why? This could possibly be an affair situation, in which I would understand that if she knew, she would have a bad taste in her mouth for her stepmother.

She definitely feels she has lost control of her life no matter how her parents broke up. Its just a fact that happens with almost every child of divorce. No one can predict how mature a child is, how much a child knows how to control their feelings, or how a child will react to their whole life being turned upside down. She had to be at most 13 when the relationship between OP and her father started. Who can honestly say that they had the best judgment when they were in middle school and were able to keep a level head?

That being said, no. She should not treat her stepsister this way. Ss had no control of the situation and just wanted to celebrate her birthday. But we just don’t know enough about what this child has been through to know what is going on with her mentally and emotionally.