title: Wordpress Dropdown that shows ONLY sibling categories
body: <p>I have a dropdown displaying all the blog categories and their children. Now my coworker thinks it would be a good idea to filter to display ONLY sibling categories. Not parent categories or child categories from other parents. ONLY sibling categories.</p>
  
  <p>Ive dug around found literally nothing, so I am opening this thread.</p>
  
  <p>things I have already tried: walker classes. this sounds promising but since the WP documentation has literally nothing about it, it is hard to know if that would work or not. im leaning towards not but again, its hard to say.</p>
  
  <pre><code> &lt;?php wp_dropdown_categories( 'show_option_none=Select a Category&amp;hierarchical=1' ); ?&gt;
   &lt;script type="text/javascript"&gt;
   &lt;!--
   var dropdown = document.getElementById("cat");
   function onCatChange() {
   if ( dropdown.options[dropdown.selectedIndex].value &gt; 0 ) {
   location.href = "&lt;?php echo esc_url( home_url( '/' ) ); ?&gt;?cat="+dropdown.options[dropdown.selectedIndex].value;
   }
   }
   dropdown.onchange = onCatChange;
   --&gt;
   &lt;/script&gt;
  </code></pre>