User blog comment:R. Lawton/How strong are some the GMG characters in comparison to Fairy Tail/@comment-5894890-20130911002816/@comment-7418318-20130911153830

So we ever learn who of themw as the older siblings?

Either way judging from their looks as children they can't have been more than three years apart, which still would leave Kagura at four years (physically) older than Erza after the timeskip and thus in her mid-twenties.