I would welcome that idea Happy Endings! Mal’s story was full of potential but they killed her off, but bringing a sister to get revenger for the curse and that is a logical result of what went before. It would let then get back to Auror’s truncated tale, maybe get flashbacks with Mal’s frenemy story with Regina. So much richness of previously dropped breadcrumbs that it feels like a story they planned rather than some weird out of left field plot veer of a sister that was never mentioned or hinted at.
Sweetgrass–Cora explicitly said any child she had would not be Rumples. This was not about Xavier expecting Cora to be untouched but Cora was not going to fulfill her bargain with Rumple because he could only claim a child he fathered by her under the changed deal. If he were the father, she would have to give him the baby and we know that didn’t happen.
That’s not to say Mal, & WW could not have been cousins to Regina, give them excuse to bring Rose back as young Cora with three girls playing together in the background or something while she and Rumple spar about something, maybe the Globe she gives him later, that he eventually uses to find Bae. Maybe Cora did something and put the blame on Aurora’s mother, and that’s why Mal cursed her in the first place, then went after Aurora?
Anyway, If they do this thing of Mal & Wicked being sister, it would restore some faith for me in the storytelling if they pick up all these dropped things going all the way back to Season 1 with Mal and Regina, who made the curse, S2 about Aurora’s story, the wraith and all this.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass