Genevieve Klimczak died in 2012 at McHenry Villa in suburban McHenry.
The Alzheimer's patient walked out of a self-locking door and she could not get back in, according to her family.
Her body was found the next morning.
"She will be dearly missing by a lot of people. We just never thought that it would end this way," her nephew Donald Lorenz said.
The lawsuit named McHenry Villa and a caregiver service called "Home Instead".
They have not yet commented about the lawsuit.