Is it a social faux-pas to mix kin and fandom while talking to other kin from the same universe?

Nah, you can totally talk fandom with other kin. 

When I say its a social faux pas to mix kin and fandom, what I mean is don’t barge into fandom spaces and announce that you’re kin. And *especially* don’t tell fandom what they can and can’t do/believe based on your kin memories/feelings.