If two pages are "related", neither one is the child, so neither page is the parent.

(En anglais, "parent" veut dire "pere ou mere", exclusivement.)

Or maybe you mean that if a page has both a parent and a "related" page, then the parent page doesn't show up on your profile.

Possibly, but there are times when this happens even without a "related" page.
Example:
http://www.summitpost.org/album/207615/ ... Flora.html
Lots of parents, no "related" pages, but no parents listed on my profile.