I don’t think Snow’s mom and Cora are sisters because otherwise that would mean that Cora wanted Regina to marry her uncle. I know Cora’s messed up but I don’t think she’s that messed up. Also this is a family show, and Regina marrying her uncle would be just plain wrong.
My theory is that Cora was a maid for Snow’s mom.
Maids weren’t too important so it is likely nobody would remember Cora. She was just there to serve, if she couldn’t do it, then they’re were plenty of other maids who could. We know Cora is ambitious so she probably wasn’t satisfied with being a servant, she wanted to be the one being served. If Snow’s mom died then Cora may have lost her job. Without Cora’s income, Cora and her miller father may have been struggling to get by. They were getting desperate and this is where Rumple enters the picture with his ability to recognise a desperate soul. Perhaps Rumple helps Cora and her father get back on their feet by spinning straw into gold for a price. Rumple may then forbid Cora and her father from ever speaking of his involvement like how Charming’s parents weren’t allowed to speak of their deal. News begins to travel about Cora and her father’s new wealth and attracts the attention of Henry Senior’s father. When asked how the straw was turned into gold, unable to tell of Rumple’s involvement, Cora’s father lies and says Cora did it. Henry Senior’s father takes Cora back to his castle and then does the tests like in the original story and Rumple appears etc etc.
Henry Senior knows of Rumple but did not know his name so it is unlikely that Cora sent out servants across the land to find out his name.
All magic comes with a price!
Keeper of Felix