Same problem here, Tarquin is nowhere to be found in my game. Are you guys undead also? Maybe undead can't do All in the family quest.
I play as lizard and Tarquin didn't show at Stonegarden. I found him in the ship.
I also have problem with quest The Reluctant Servants, when i talk with Farimah i don't have options to start this quest. Maybe I need to do something to get this options?
I find info about this quest from following site (maybe this page misleading me)
http://divinityoriginalsin2.wiki.fextralife.com/The+Reluctant+Servants
