Zap2ItTV has some pages recapping the relationships of the characters, taking into account the possibility that Regina is Rumplestiltskin's daughter. The perils of open-ended serialization become very apparent. The website plays it perfectly straight, but the upshot is still a great deal of humor at the show's expense.
As for the likelihood of a heretofore unknown paternity for Regina? The distinct implication was that Cora reneged on consummating the deal. But when did Cora learn to appreciate the value of appearing to be someone else?