Menu with siblings and all upper-level links

Hello,
I was asked to do a specific menu for a website and I have no idea how to do it.

What I want :
– The menu is different depending of the page
– It shows the top-level page and all of the 2nd level pages
– It show all of the sibblings of the current page
Example here:
https://www.univ-lille3.fr/universite/presentation/
The menu at the left of the content is exactly what I need. There is a title which is the top-level parent, and then a list of the 2nd level pages, and when you enters a page, you see the childrens of that page. When on a children, you still see the sibblings.
As the menu is different depending the page we’re on (each top-page having its own subpages), I’m using php to generate that menu.

Tested solution 1:
I got a code to get all of the subpages (on any level) of my top-level page. Now I have to find a way to hide the unwanted items and keep the one I want to keep.
I’m using this code:

<?php
  if($post->post_parent)
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  else
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  if ($children) { ?>
  <ul>
  <?php echo $children; ?>
  </ul>
  <?php } ?>

So I get a complete hierarchy of my top-page. Using display:none, I can hide the unwanted children, and using the current-item class, I can display the item I’m on.
Problem:
I don’t find a way to get the sibblings as well.
I was hoping for a CSS selector that would be like “every ul that containts a .curent-item” but it doesn’t exists. Maybe I should do some javascript to add a class to siblings or the ul containing curent-item but I have no idea how it works. I only know basis about such languages.

Tested solution 2:
Another code I found on the codex (I can only copy/paste codes because I don’t know how to code)

<?php
$ancestor_id=6;//this code is wrong
$descendants = get_pages(array('child_of' => $ancestor_id));
$incl = "";

foreach ($descendants as $page) {
   if (($page->post_parent == $ancestor_id) ||
       ($page->post_parent == $post->post_parent) ||
       ($page->post_parent == $post->ID))
   {
      $incl .= $page->ID . ",";
   }
}?>

<ul>
   <?php wp_list_pages(array("child_of" => $ancestor_id, "include" => $incl, "link_before" => "", "title_li" => "", "sort_column" => "menu_order"));?>
</ul>

Seems to work nice for the hierarchy, I have the needed pages. But I get a perfectly flat list. All items are li, and there is no inclusion. Children are not a sub-list. Then, my children are in the top of the list, with their parent below. I have no idea how to style this in a clean way to get the parents above, and the children listed below with a different style showing they’re children.

Tested solution 3:

function menu_rubrique($pageid=null){
	global $post;
	if($pageid == null)
		$pageid = $post->ID; //Si il n'y a pas d'id, on récupère celui de l'objet.
	$current = get_page($pageid);
	if($current->post_parent != 0){ //Si il y a un parent
		$ancestor_id=$current->post_parent;
		$ancestor = get_page($ancestor_id);
		$descendants = get_pages(array('child_of' => $ancestor_id, 'sort_column'  => 'menu_order, post_title'));
		$incl = "";

		foreach ($descendants as $page) {
		   if (($page->post_parent != $ancestor_id) &&
			   ($page->post_parent != $post->post_parent) &&
			   ($page->post_parent != $post->ID))
		   {
			  $incl .= $page->ID . ",";
		   }
		}?>
			<ul id='menu-rubrique'><li><a href='<?php echo get_permalink($ancestor_id); ?>'><?php echo $ancestor->post_title; ?></a>
				<?php wp_list_pages(array("child_of" => $ancestor_id, "exclude" => $incl, "link_before" => "", "title_li" => ""));?>
			</li></ul><!-- #menu-rubrique -->

	}
}

I don’t even remember where I found this.
Problem:
This one give me a nested list, but the problem is that it changes when I go on a child page. I get the page’s siblings and children, but not the parents.

So…. I tried several solutions but none of them give me the wanted result. As I don’t know much how coding works, I can’t edit the codes to get the result I want.
That’s why I’m asking to you. If you know the way to edit any of those codes to help me get what I want, or if you know any other code or solution that may help, I want it.

Thanks a lot for any help.
Hoping my post isn’t too long…

PS : I’m not English, then I do lots of mistakes when writing. I’m sorry for this. I hope everything is understandable.