This is because the relatives technically an other type/class of characters (has common, relative, and creature type/class). Thats why some commands work with them differently (like the query based ones). 'Tags' are just 'marks', or 'notes' whats works differently (can be added, modified, deleted on every type of npc type/class without any restricition or consequence). So you can give any common NPC a "secretary" tag, the script's commands will load them when it runs, but isn't will work on the non-common types.Is it a bad idea to work with relatives? It's forbidden to add relatives to your business. And there is a bug that "secretary" tag doesn't work to relatives. If you recommend your relatives, they work in the same place as you, but they aren't your collegues.
That recommendation script is wrong at point to let relatives to be added because isn't implemented on the relative type NPC's have that colleague (neither the employee) status (by design).