Year long late reply, but, the only reason people speculated gwynevere to be priscilla's mom was because her illusion was the only one seemingly possible to mate with a giant dragon. But take note that in dark souls 1, her appearance was just that, an illusion. And the logic that the theory was built upon was really flawed.



The other pertaining theory was that Yorshka from DS3 was the counterpart of Priscilla of DS1, which would make Priscilla Gwynevere's youngest sister.