I guess you have looked at it, but place a link to the related_objects fetch doc page here anyway: http://ez.no/doc/ez_publish/technical_manual/4_0/reference/modules/content/fetch_functions/related_objects
And while on it, the different type of relations in ezp: http://ez.no/doc/ez_publish/technical_manual/4_0/concepts_and_basics/content_management/object_relations