Anyway, as for evidence.
We now know Henry sr. was a prince, who lost his claim to the throne somehow- check
Cora would need motive for hating and wanting revenge on her sister (family intrigues are always the worst and most complicated ones, hence- a reason for Snow and her father the King not knowing Cora and Regina in Stable boy-check) But her sister ‘having it all’, everything Cora ever wanted. Would be motive for jealousy- check
I agree with this except I don’t think that automatically means they are sisters. I think Cora just really wants a throne (any throne) back. And she’s willing for it to be Regina because I think we all know that Cora would be the true power behind the throne. She could kill Eva without it being a family intrigue. We know Cora arranged for Snow’s horse to go go crazy in Stable Boy thus leading for Regina to save SW and met Leopold, so I don’t think it’s far outside the realm of possibility that she’s been plotting to take the throne for much longer.
If Henry did lose his kingdom/throne to George, it wouldn’t make sense for her to go after that realm because George had an heir, the original James who would have been the same age as Snow, and thus not marriageable age for Regina. Killing George wouldn’t help Cora get a throne.
I suppose she could have arranged for Regina to marry George but that wouldn’t be faithful to the original Fairy Tale.