i want to get the parent id, only on parent -> child -> child page

The page object has 2 properties: post_parent which is the parent page, and ancestors, which is array of all of the page’s ancestors in the tree.

 $currentPageId = get_the_ID();
 $page = get_page($currentPageId);
 if (in_array(34, $page->ancestors)) {
      $siblings = get_pages(array('child_of' => $page->post_parent));
 }