I doubt that the Queen of Hearts is Regina’s older sister. In the original story of Rumplestiltskin, the miller’s daughter was able to keep her first-born, because she sent her servant to spy and discover the imp’s true name. So, Rumple was in fact never able to collect on his deal, so to speak. He never received the “payment” he thought he was owed. It makes me think that he has been after Regina ever since, because Regina was the first-born that got away.
Also, I doubt that Bae is Cora’s child, just because Bae lived centuries before Cora was born. Even if Rumple granted Cora an extra long life, it wouldn’t explain a key point. Rumplestiltskin made the deal with the miller’s daughter after he became the dark one. Bae was born before when Rumple was still just a regular man. Also, I don’t think Cora and Rumple were ever married. In the story of the miller’s daughter, she marries the king, not the imp.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy