Change Content region width when Sidebar has content?
$page['sidebar_first'] is an empty array when you have no content in that region. So it is defined and the condition 

if ($page['sidebar_first']):


is true even if you have no block in the sidebar. So 

print render($page['sidebar_first'])


gets executed and, after the rendering, $page['sidebar_first'] will look something like

array(
  '#printed' =&gt; true,
  '#children' =&gt; '',
)


meaning that it's no longer empty and the condition if (!empty($page['sidebar_first']): will be met in any case. You may want to rewrite your code to something like

      &lt;?php if (!empty($page['sidebar_first']): ?&gt;
       &lt;aside id="sidebar-first" role="complementary" class="sidebar clearfix"&gt;
         &lt;?php print render($page['sidebar_first']); ?&gt;
       &lt;/aside&gt; &lt;!-- /#sidebar-first --&gt;
       &lt;div class="content-region-sidebar-sibling"&gt;
         &lt;?php print render($page['content']); ?&gt;
       &lt;/div&gt;
     &lt;?php else: ?&gt;
       &lt;?php print render($page['content']); ?&gt;
     &lt;?php endif; ?&gt;