Peterson asked the 3rd District Appellate Court last month to reverse a lower-court ruling that gave control of Savio's estate to her father and sister. Those family members wanted her estate reopened and a new executor appointed.
In the appellate decision last week, Justice Robert Carver wrote that Peterson isn't a suitable person to serve as executor or to choose one. Peterson has been named a suspect in the disappearance of his fourth wife, Stacy. He's denied any wrongdoing.
Peterson's attorney, Joel Brodsky, says he might appeal the justices' opinion to the Illinois Supreme Court.
Savio's death has been ruled a homicide.