There can be numorous reasons why they wouldn’t necessarily see eatchother even though they were sisters and so that Snow and her father wouldn’t know them.
Example: Cora was the oldest and went of as the first one to marry, she found Prince Henry and then the ‘spinning gold for the King’ story happent. So the King and Prince (and Cora) lost the kingdom and their fortune to Rumple, Cora felt guilty and furious, she might have tried to get help and sympathy from her family and her sister, but not gotten it. Then when her sister goes and marries a prince and finds happiness she is jealous.
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