Relatives are requesting a paternity test to possibly match DiMaggio, suspected of kidnapping and murder, to Hannah Anderson and her brother, Ethan.
"We are requesting DNA samples from Hannah and anything they can get from Ethan," family spokesman Andrew Spanswick told CNN affiliate KGTV. "There are rumors Jim was the children's real father. The parents didn't marry until 2002."
DiMaggio, 40, was killed during a shootout on Aug. 10 in the Idaho wilderness when police rescued Hannah. The teen's brother and mother, 44-year-old Christina Anderson, were found dead six days earlier in DiMaggio's burning house outside San Diego.
A representative of the Anderson family rejected the theory.
“Brett and Tina Anderson did not meet Mr. DiMaggio until the sixth month of Tina’s pregnancy with Hannah. Brett Anderson’s DNA was used to identify the body of his dead son Ethan Anderson,” the family said in a statement.
DiMaggio also left $112,000 to Hannah Anderson's paternal grandmother, Bernice Anderson.
"We think it's strange he left them so much money with no explanation," Spanswick said, according to USA Today.
Hannah and Ethan's father, Brett Anderson, lives in Tennessee.