You can just access the $eCart1->Items array directly and sort it in place, or save it and sort it separately.
You could use something like:
<?php
$sortedArray = $eCart1->Items;
function cartSort($a,$b) {
return ($a->ParentSKU > $b->ParentSKU)?-1:1;
}
usort($sortedArray, "cartSort");
?>
Then the $sortedArray would be sorted by the ParentSKU.