In the Brady Bunch (the TV show, not the film series), the title song establishes that the 6 new step-siblings are from Mike and Carol's former marriages (or partnerships).
What happened to Mike's former wife / partner?
What happened to Carol's former husband / partner?
Were these two others ever mentioned for any amount during any episodes, or if still alive, did they ever make any appearances or claims towards their children? I have never watched the pilot episode / first episode, so I do not know if that episode explains it.