Having discovered a whole branch of new, distant relatives, I wanted to share my tree with them so badly, that I sent one of them an invitation to join my tree as to my brother (that she certainly is not). She registered, using that invitation and now, when editing the tree, appears as my brother, although due to her help there is a profile showing her on her own in our shared tree.
Is there any possibility for her to change her "home-profile" from my brother to herself within the tree so the relationships and other features would appear as they are in reality to her?