Board Thread:General Discussion/@comment-26509198-20160713214402/@comment-24674051-20160715173743

I think the girls were supposed to be 10 and 12. When Zelena was born, Cora was working as a barmaid, and seemed to have been somewhat experienced at it. So I'd put her around 19-21 for Zelena. Which would put her at 21-24 for Regina. So by Sisters, Cora would have been definitely early 30's. And by the time of the DC, assuming that Regina was around 30, Cora would be in her 50's.

As for the casting, Rose handled the role for "Bleeding Through" and "The Miller's Daughter", which were the late teen/early twenties Cora. Barbara handling the 30-something Cora is not unreasonable. Besides which, as others have pointed out, Barbara was available, and the show probably had exhausted it's budget for additional recurring actors.