Also known as the Magic or Madness trilogy by Justine Larbalestier, consisting of Magic or Madness, Magic Lessons, and Magic's Child. Too bad I like the more fanciful working title better, huh?
In Reason Cansino's world, magic doesn't exist. Reason and her mother have been running from "the witch" (Reason's grandmother) for her entire life. When her mother finally snaps, Granny Esmeralda takes Reason in... and Reason makes the obvious discovery. The dilemma is this: with magic, there are two choices; you can use it and die young (like all the twenty-somethings buried in the Cansino's graveyard) or avoid it and go insane (like Serafina, Reason's mother).
The first book was okay, although it ends on a cliffhanger. The Australian dialect adds some nice flavor ("chunder" has to be one of my favorite words ever). However, the story is perhaps a bit goofy at times, perhaps because it's classified as young adult. The second book contains my own personal dilemma: is a magical compulsion to have sex with someone rape? Then I would call it rape... but the author never even mentions the word or even says in any clear way that what happened was wrong.
So, this is a cute young adult trilogy, and apparently suitably entertaining (my younger sister read through the books very quickly), but I wouldn't say it was truly great. On the plus side, the ending was a surprise, but the level of writing was not very sophisticated, and the author avoided discussing some of the tougher issues that came up. If you don't already enjoy fantasy or teenaged coming-of-age stories, it would probably be better to stay away.