I remember reading it when I was younger. It definitely came out after 2000, but probably before 2010. It was the first one in a series. The main character (young, 12-ish, female) is a sort of outcast because she can't do magic like everyone else that she knows, but only through spelling out words (or something like that).
It was definitely a paperback, but the cover (or at least one of the covers) may have been purple? This is even more fuzzy, but it may have had the character and her older sister together (it's been so long since I read it.