Okay, a few things strike me as a little off here.
One, Sunset Shimmer is an ex student of Celestia and therefore is probably Twilight's age or older. Fifteen seems a little young. Did you misunderstand and put your own age instead of the character's? I'd just put "young adult" there, if I were you.
Two, Sunset Shimmer is the antagonist of EqG. I have not had the opportunity to see the movie yet, but I imagine that means she isn't just "nice and kind and sweet." Antagonists usually have darker characteristics, too. Being resentful, for example.
That backstory is really weak. Tell us what actually happened.
As for the role play post, you should only be controlling your own character. In a situation like that, someone else would be playing Trixie, so you shouldn't specify Trixie's actions. Your sole concern should be Sunset Shimmer's actions.