I’ve always just figured that Snow and Red’s sisterly bond was their way of acknowledging the story of Snow White and Rose Red. Mind you, the Onceiverse has mother and daughter Sleeping Beauties, so there’s no reason why there couldn’t also be mother and daughter Snow Whites.
I’ve entertained the idea before that Eva and Cora could have been sisters. If, as many have speculated, Cora is the one who killed Snow’s mother, there’s gotta be some reason for that, and the jealous sister angle could work quite nicely.