Ledger died in January at the age of 28. At that time, he was publicly known to have been the father of 2-year-old Matilda, his daughter with his ex-girlfriend, actress Michelle Williams.
The actor's parents have declined to comment on reports Ledger also fathered an older daughter, Britain's Telegraph said Monday.
The newspaper said the mother of the older child discovered she was pregnant after her affair with Ledger had ended and she was living with another man.
"There is a very real possibility that Heath was the father," the actor's uncle, Haydn Ledger, told Sydney's Daily Telegraph.
Haydn Ledger has been locked in a bitter feud with his brother Kim, Heath's father, and has ignored pleas from the family not to speak to the media about Heath's death.
"(The former girlfriend) had the baby. Everyone lived under the assumption that she was the daughter of the mother's boyfriend and that is how she has been brought up," a second family member told the newspaper.