HTML code of sidebar varies according to plugin. Therefore, it is difficult to cope at a time. However, if you have knowledge of Javascript, you may be able to cope by making plugin of javascript OR jQuery.
If it is only related question, hack below is effective.
// replace [start]
//$themeobject->output(
// '<div class="qa-related-qs">',
// '<h2 style="margin-top:0; padding-top:0;">',
// $titlehtml,
// '</h2>'
//);
$themeobject->output('<div class="qa-related-qs">');
// replace [end]
P.S (This is just reference).
Purposes are different, but I made similar plugin in the past.