While, I do think you brought up valid points in the video, I do think that as you put, Sweetie is growing up and changing. Something else to mull over is that perhaps Sweetie Belle has had these feelings of living under Rarity's shadow for some time now and the costumes was the thing that pushed her over the edge. True, we have never seen this brought up before but that doesn't mean, Sweetie Belle didn't have these feelings. She could have been keeping them bottled up and she just exploded on Rarity here. It was not the best way to handle things but a lot of us have been in situation not unlike what Sweetie Belle was feeling.