We were discussing it in another thread. Possible scenario could be that Cora and Snow’s mother were sisters and rivals at the same time. Cora lost this rivalry and cursed Snow’s mother and her family (or something), the rage automatically got transferred onto Regina who could subconsciously hate Snow from the start but didn’t find an excuse until Daniel’s incident.
Another cool theory is that Snow’s mother was the White Queen from Wonderland which again would bring rivalry between her and the Queen of Hearts and would explain Cora’s connections to that land.