In Once Upon a Time season 1 episode 14, the plotline revolves around "the nuns" trying to sell candles, and one of the main stories is Leroy/Grumpy falling for Sister Astrid/Nova the fairy. When Mary Margaret/Snow White finds out about this, she has exactly the reaction one would expect about a man falling for a nun. (Basically: "She's a nun. What part of unavailable did you fail to understand?")
However, we've repeatedly heard people refer to Snow herself as Sister Mary Margaret, but nobody bats an eyelash at the fact that she's in love with David/Prince Charming. Well, they do, but only because she's seen as a homewrecker, not because she's a nun who broke her vows.
So, what gives? Exactly what is Mary Margaret's occupation in Storybrook?