Just watched the episode finally. I was surprised they spilled that sisters secret so early in the season.
Zelena did say Cora had her before she got involved with the royals, so I think that automatically takes out Rumple as a possible love child between him and Cora. Still, if the green skin isn’t genetic, that only other possibility, is that she was cursed green?
New theory about Zelena’s baby daddy, this would be twisted but typical Once, her father is Leopold, Snow’s dad. Which, we already know that we will be seeing him again this season as a young man, courting Eva but now, I’m thinking, it will be a similar situation. Where Cora probably had an affair with Leopold, like maybe he was a playboy prince and Zelena was the result of a one-night stand or something. But something changed him and became the good king, probably falling for Eva.
It would also explain an upcoming title, Bleeding Through. Which could also mean, blood ties between multiple families. Snow’s family (through her father) and Cora, from Regina..
Note: if my theory has already been talked about, please feel free to ignore me. Lol.