Custom Nav Walker to show siblings and children of current branch?

I have seen very similar (if not exact) queries to this all over Google, but always seem to find something short of what I need. I am trying to create my full vertical menu using wp_nav_menu in my template. Only I would like to only show the current branch, siblings, and their children. Kind of an accordion-style menu. I prefer to do this in PHP, not using CSS or JQuery.

Example full menu:

- Home
- Products
   - Cool Item 1
      - Item 1 Details
      - Item 1 Gallery
   - Cool Item 2
      - Item 2 Details
      - Item 2 Gallery
- Services
   - Svc 1
   - Svc 2

If, for example, I am on current item “Cool Item 1”, I want to see all Root menu items, all Products, but only children of “Cool Item 1”, no children of “Cool Item 2” or any Services.
Like below:

- Home
- Products
   - Cool Item 1 (-current page-)
      - Item 1 Details
      - Item 1 Gallery
   - Cool Item 2
- Services

If I click on Services, I want to see:

- Home
- Products
- Services (-current page-)
   - Svc 1
   - Svc 2

Currently in my custom walker I have the star_el method like so: (make_menu_item is typical menu display you’d see searching walkers)

function start_el(&$output, $item, $depth, $args) {

    // am I on the current Item?
    if($item->current_item_ancestor or $item->current_item_parent or $item->current){
        $cur_branch = true;
    }
    // this works for all current branch items, but not siblings or children
    if( $cur_branch || $depth == 0 ) {
        $output .= $this->make_menu_item($item, $depth, $args);
    } else  {
        // need to know if my parents are in current branch, then OK to show children
        // not sure how to do that from this point?
        if($item->menu_item_parent && $depth > 0) {
            // this currently shows everything, not what I want
            // if this is removed, no children objects show, even under cur_branch
            $output .= $this->make_menu_item($item, $depth, $args);
        }
    }

}

I know this logic probably isn’t the best, but its a start. I can easily tell if I am in the “current branch” of the menu, and I have setup $item->hasChildren in the display_elements method, but hasChildren doesn’t do me any good for children of current. It would be most helpful I just knew at this point whether or not my parent object is in the current branch, because I could either show the menu item or not then, which is what I want.

I could do an SQL query, but I sure rather not do that. Is there a way to get parent info in the walker array’s and objects available ($item or $elements?) ? Maybe I am going about this all wrong? Seems like it should be easy to get parent info, but calling get_ancestors() here is giving me an empty array.

Thank you for any help!
Greg