$('li[rel=7]').siblings().andSelf();
// or:
$('li[rel=7]').parent().children();
Now that you added that comment explaining that you want to "form an array of rels per column", you should do this:
var rels = [];
$('ul').each(function() {
var localRels = [];
$(this).find('li').each(function(){
localRels.push( $(this).attr('rel') );
});
rels.push(localRels);
});