I had mixed, non consistent, results. Some non-played sims got their tombstone in Mausoleum, despite not being met ever by any of my playables (Newspaper: "ohnoes, Algernon Whatthefuck has passed away"; Playable sim: "... who?"); some others, best friends, disappeared without a trace.
This has been the exact case for me too. In fact, my sim's own mother wasn't added when she died and now she is lost for the forseeble future. Yet, some random freak who I had never come across was added instead.