Evanston police Cmdr. Tom Guenther says the bodies were found Friday after authorities were called by a senior advocate.
Autopsies were planned but Guenther said police do not suspect foul play.
The 90-year-old woman's was taken to a hospital for observation. Her identity was not released.
The Cook County Medical Examiner's office identified the dead as Anita Bernstorff, who was last seen alive in May 2008, Frank Bernstorff, not seen alive since 2003, and Elaine Bernstorff, who was last seen alive in the early 1980s.
Anita Bernstorff was the eldest, born in 1910.