Due to LIghtning Dust's reckless actions at the academy , she had been kicked out because of the incident involving that tornado and nearly killing Rainbow Dash's friends in the process. If it hadnt been for the other cadets saving, they could of been killed or worse. Now she finds out that her older brother Sky Stinger, just got in because of both Twilight and Rainbow Dash seeing and helping him with his confidence issue.
If he can get in, maybe she could be given another chance. That was if she could swallow her pride and confront Dash without getting a Black eye out of it.