Siblings panel seems to display children when displayed for a non-terminal page

The siblings panel, introduced for XWIKI-12599, displays the tree rooted in the current page's "space" (parent in the sense of the DocumentReference sense), which displays the actual children of the current page, not its siblings; actually, it's the siblings of the WebHome page of a page tree, which are actually its children).

More specifically, if you do the following:

• create page A, non terminal
• as children of page A, create pages B and C, non terminal as well
• as a child of page B, create page D, non terminal
  • at this point, the pages with the following references should be present on your wiki: A.WebHome, A.B.WebHome, A.C.WebHome, A.B.D.WebHome
• go to the administration of page A and configure the panel "Sibling pages" to be displayed on the page tree, on the left or right, it doesn't matter
• navigate to page B

Expected result:

• sibling pages panel should display pages B and C (with page B being selected, optional)
• page B should be unfoldable to reveal page D

Actual result:

• sibling pages panel displays page D (the child of B)