Try this:
<?php
if ( '0' != $post->post_parent) {
include 'siblings.php';
}
?>
!== refers to an exact type match too.
So your code doesn’t work because the post id is stored as an integer.
!= will disregard the fact that you are comparing a string to a number.
Hope this helps.
You could also simply use the integer 0 in place of '0' in the first place…