Post Children OR siblings, not both

I am using one navigation script on my site. I want to have the code to say (assuming it is on a level two page)…. “list my children”…. if it is a level three page (meaning you clicked a child link) it then says… “list my siblings”.
Make sense?

I have the children part ok, but need help finding the parents children (siblings)..

$children = get_pages('child_of='.$post->ID);
if(count($children) != 0) {
   wp_list_pages('title_li=&depth=1&child_of='.$post->ID.'');
}

else{.....GET SIBLINGS.....}

Thoughts?