sibling taxonomy terms
You can try using taxonomy_get_children($termid) to get children of term and then taxonomy_get_parents($termid) and again taxonomy_get_children() to get the siblings . 
The following is the code I used in such situation :

&lt;?php

if (isset($page['#views_contextual_links_info']['views_ui']['view']-&gt;args[0])){
    $termid = $page['#views_contextual_links_info']['views_ui']['view']-&gt;args[0];
    $terms = taxonomy_get_children($termid);

    if (empty($terms)) {
        $parent = taxonomy_get_parents($termid);
        $terms = taxonomy_get_children(key($parent));
    }
}

if (isset($node-&gt;field_category['und'][0]['tid'])) {
    $termid =  $node-&gt;field_category['und'][0]['tid'];
    $terms = taxonomy_get_children($termid);
}

?&gt;