Ok, please don’t take this personally as we are discussing a theory here and nothing personal.
As I’ve said before, The Stable Boy makes it fairly obvious there is no such relationship. When looking at theories, I go back to the episodes, interviews, tweets, etc., and look for things. There is nothing other than one episode showing they were strangers.
As for Cora being evil from the start, K&H have stated in a recent interview that Cora was not evil from the beginning. They frequently say that “evil is not born, it is made”. Something happened to Cora to turn her. I suspect it all has to do with Rumple, reneging on turning over baby Reggie and losing Henry’s Kingdom. I also suspect, that she had a hand in Snow’s mother’s death (or seemingly death) so as to make room for Reggie. But I don’t see anything to lead us to believe they were sisters.
Keeper of Pandora's Box & The Yellow Brick Road.