I belive there's a word for this, as I've heard it before, but I can't think of it. If a sibling comes many years after the rest of the pack, what is set sibling called? It doesn't need to be one word, but just something you can call someone.
If this helps, I've heard siblings like that are a product of the mother missing having young kids around, due to only having older children, hence having one many years later.