On March 17 Wetpaint shares a quick interview with Lana Parrilla, who plays Regina the Evil Queen on "Once Upon a Time." In the interview, she reveals that even though they are half-sisters, they have very different intentions.
Regina's intentions have always been that she just wants to be happy with a family. She doesn't want to live without Henry, and will do anything to find someone that loves her for who she is. She has proven that she will do anything to make that happen, even if that means killing Snow White who she feels stole that happiness from her.
Zelena is green with jealousy, quite literally, because Cora gave her up but kept Regina as a child. She won't stop until everyone is just as unhappy as she is and is making quick work of it all. Regina has never done anything to Zelena, in fact, she didn't even know she existed until she found her living in her castle. However, Zelena is still angry that she was the one Cora kept and "gave her everything" which she is envious of.
Why Zelena is holding this against Regina is completely unknown since it wasn't her decision and Regina would likely have been better off and preferred that Cora had given her up like she did Zelena.
Will the Wicked Witch ever be happy, even if she makes everyone miserable around her?