Get child/parent categorys from category number

This code without echo, will show all categories in variable that holds ‘9’ as the id number.
Note: Will only show category with posts. (make a test one to show it).

Get child of
wp_list_categories('child_of=' . $mycategory_id . '&title_li=');

Get parent of
wp_list_categories('parent_of=' . $mycategory_id . '&title_li=');