I like your theory but I don’t know about her being a maid, because one of the only known facts about Cora is she was the millers daughter, which would make you assume she worked in a mill.
And even if Cora and Snow’s mom were sisters that wouldn’t really be Regina’s uncle in blood sense. She would have been related to him only through marriage, so even know it is weird and odd, it wouldn’t be unthinkable. I don’t know if I believe or even want these two to be sisters or even related but I would be shocked if these two have no history
Keeper of Swanfire's dream catcher, the Blue Fairy Plot Device, the contact name "Her", the lanyards, the trigger, the Netherworld room and necklace, Charming's quote on Thanksgiving, and PP's pic of Henry