Well in the episode, "Baby Cakes" Twilight and Rarity state that unicorn magic can come and go. And in the episode, "Babs seed" we see sweetie belle give a few sparks from her horn. And in the season 4 premiere, sweetie belle is having trouble controlling her magic because of the whole everfree issue. And sense rarity was having a similar issue, I can assume that sweetie belle did indeed know some magic.