I've noticed the game seems to think Sims that aren't in the same household aren't in the same family. For example, when my Sim's father died, she wasn't affected at all by it because he didn't live in the same household.
I don't know if that's a bug or by design, but it's a lot different than in real life! Usually parents' deaths are a very traumatic event (assume they aren't totally estranged...)