AITA for refusing to apologize to my stepsister for stuff that happened 12 years ago?

The story revolves around a person who, as a child, struggled with their widowed father’s new relationship with their now-stepmother and stepsister. The individual expressed anger and resistance to the new family dynamic, prompting therapy for all involved.

Over time, the family situation improved, but the individual still harbors negative feelings towards their stepsister, who seems dismissive of their grief and insensitive to their past struggles. When the stepsister learned about the individual’s initial rejection of the family, she demanded an apology, leading to conflict. Despite pressure from their stepmother and accusations of immaturity, the individual refuses to apologize, leading to further tension within the family dynamic.

Here were the top rated comments from readers in response to the OP’s post:

The OP responded here:

The OP again responded:

So, who do you think is in the wrong here? If you could give the OP or her stepsister any advice, what would you say?