The traits thing is for up to 4. I don't see new traits being piled on Willy the Conker, since he starts with 7 of them none gets added. So the only thing needed for this is to add up to the defined number of traits to historical chars.
Guess I just don't see what you'd want a trait that acts like that for, or why you wouldn't just tie it to the title itself.
Oh, and:
More specific familial relationship opinion modifiers (E.G., brothers, uncles, grandparents)
This can be done by using smart triggers in a setting event that is ran on an on_action pulse, setting an event_modifier