I think the blood magic, the fact that Cora is definitely Zelena’s biological parent, and the fact that the show really hasn’t even suggested that Regina was anyone else’s–plus the fact that Cora was a princess when Regina was concieved and born (so, serious lack of privacy) all make a very strong case that Regina IS Cora’s daughter, biologically.
It makes for an interesting AU theory, but so much of the plot has hinged upon this relationship being accurately presented up to this point that I don’t think they’d pull a reversal that…drastic on us. Also, the marriage to Leopold was messed up even before we were told he was once engaged to Regina’s mother…having her be Snow’s sister (even half) is…kinda a step too far. (And the havoc on the family tree? Do you REALLY want it to be more impossible to untangle?)
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing