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I think it was very immature of her to treat you that way, but the kind of immaturity that goes with her age. Hopefully as people leave their teen years they see the hurtfulness of this behavior.
At this point she has dug herself into her position and it is unlikely that she will apologize to you. Perhaps you can talk to her parents (well, the one you are related to by blood) and have them explain to your great niece just how wrong she was.
You may not 'fix' this in time to attend her graduation, but it may bump her into some better behavior for her future.
Thanks again.Hugss.Jerrie