An Angel's Grace (Castiel x OC)

Alexandria, or Alex for short, is the youngest of the Winchester's. she's not their legitimate sister, but her mother died giving birth to her, and her father gave her up to the Winchester's.
She's had a strange mark on her back, a cross, for as long as she can remember. She considers it a birthmark. But there's something weird about it: It glows a bright blue.
while saving people, hunting things, and protecting her older brothers, research about her cross has consumed her completely.
She meets an angel that calls himself Castiel. He tells her what the cross means, but even he doesn't know how, or why it was put there. Something, even biggger than the Winchester's level of crazy, unravels.
However.
Alex can't help but give in.