In WordPress, pages can be hierarchical that enables you to organize them with parent and child pages.

For instance, you may have a parent page with several child pages and then a child page may have other sub child pages.

Note: by default, there is no parent page to any page.

Below is an example of child pages under a parent page.

So, if you want to query the children pages of the parent page, then choose "From Post Parent" and under Dynamic settings enable the toggle, My Children (as shown below)

Those pages which are in the same hierarchy are siblings. So, if you are to query the siblings of the current page, then enable the "My Siblings" toggle to query the data.