It's not just the venetians, it includes you.

If your royal family is dwindling for whatever reason (just started campaign, all became homosexual, assassinated, black death, etc...)
you get the "Candidate for adoption" window at new turn. same thing is happening for the venetians and it doesnt' cost money to adopt either.

Of course the geenrals bodyguard has like 450gold upkeep or something..but anyway...