AITA for not bringing my niece shopping after she was extremely rude/mean to me, even if she “apologized”?

I do not have kids, and I also have not really had a lot of experience dealing with kids before my niece. I don’t have younger siblings or cousins, I never babysat, etc. I’m explaining this because this is my blind spot and why I’m posting.

About a month ago, my niece “Gabby” and I were at the park together. I like to spend time with her and usually take her out to do things just the two of us every couple of weeks. While we were at the park, I was talking about an event I am going to attend with my boyfriend of a few years. She asked to see my dress, so I showed her a pic of me in it.

She made a “yucky” face and said “That’s soooooooo ugly. You look really fat. Isn’t [[my boyfriend]] gonna think it’s bad?”

I was so hurt. She’s only 13! I’m not even a big girl in any sense. I have fat on my body, but I am definitely not fat.

My sister was surprised and said she would talk to her. A few days later she had Gabby come over to apologize. From my POV, it was not sincere. She was rolling her eyes and looked angry and just got out the words. I told her thank you for apologizing and then told them to leave. I have not gone out my way to spend time with her since.

I had told Gabby that I would take her shopping for the summer and we’d pick out fun stuff together. I look forward to doing things like this with her. Not really anymore. When we were at my mom’s house for mother’s day, Gabby asked me when we would go.

After her stunt I’ve changed my mind. I said that she probably has plenty of summer clothes to wear. She was upset and said “But I said I was sorry!” over and over.

I told her that I know she said she was sorry but just because someone says they’re sorry to you doesn’t mean that your actions are forgiven. That you have to prove to the other person you won’t do it again, and she hasn’t.

My mom and sister say I’m expecting adult behavior from a child and that it was ridiculous to cancel the shopping date. I am apparently beefing with a child at my big age. They said that she apologized and hasn’t said anything else rude to me since. They asked how long I’m going to hold it against her.

I really don’t know how to take this. I trust their judgment most of the time. But this was just an outright mean thing from her. I am worried that as she grows up this behavior will continue and she will turn out to be a bully. AITA?