Marion, I have seen a spread of 8, 12, 15, 20+ years tween siblings, same two parents, so that would not be unusual. And it was among families with 2 to 13 children -- sometimes I paused long enough to wonder who was more surprised -- mom or pop. Um.
And mistakes and misinformation are not uncommon either, Oral History being what it is, and Mary-Joe Hickey is working with Oral History from what she writes in her post.
And Mary-Joe likely can investigate this matter some more, and she may discover this is a relative, or she may discover this is not a relative. But she will make the discoveries in due time.