I don't know if it affects sims reborn into different families. I got the familiar buff in one save when the sim was reborn into the same household as his family. In another save, when having him reborn in a different family, he didn't get a familiar buff when meeting his sister from his previous life. I would have thought that it would work for sims reborn outside their family as the buffs for each aging up implies for them to search for people in their pasts lives and "reconnecting". When I do this, however, I don't get that familiar buff nor does it appear in the view relationship tab. Not sure if this is intentional or a bug.
