the ideas

{ angel: a person of exemplary conduct or virtue. }
{ sin: an immoral act considered to be a transgression against divine law. }

Amy Morgan, a relatively well known hunter stumbles across the Winchester Brothers and realises that maybe they need her more than they let on - everyone knows that "Winchester dumb" is a well known insult in the hunting world so yeah, maybe they do need her despite what they (Dean) insist.