In personal details the form asks for the immediate family (parents, sisters, children) and afterwards it asks "have you told us about all of [name] relatives?"
I guess the answer ist "yes" if I added my parents and sister, right? I mean basically I have other relatives like aunts, grandparents and so on too. But the question if I mentioned all is related to my immediate family, right?
If you don't see the relationship in the drop-down menu when you're choosing their relationship to you then you can't list them anyway. I believe it is just immediate family, though, meaning parents and siblings.