I think that Cora is going to turn out to be the miller’s daughter from the original Rumpelstiltskin story. She was poor and made a deal with him to spin straw into gold. As payment, he asked for her firstborn- which would be Regina. Then he gave her a loop hole that she could get out of it if she guessed his name. Her servant overheard him singing his name and told her so the deal was off, and he got angry.
In an earlier episode, I got the sense that Regina had once been an apprentice of sorts to Rumple…that he may have helped her get her powers. Maybe he is responsible for Cora’s powers too.