A source close to the teenager - whose father is the late Michael Hutchence - and her family told Grazia magazine: "Thom's family are being amazing but surprisingly it is Tiger who has proved to be the greatest support for both families.
"She is the girl left in the same scene her nephew was found in and yet she has become this incredible rock for everyone when she's just a teenager. It's amazing."
Peaches' husband - with whom she also had two-year-old son Astala - and her sister Pixie, 23, are said to be finding it particularly hard to come to terms with her death, which is believed to have been caused by heroin use.
The source added: "Pixie has been in a state of uncontrollable grief - Peaches was so close to her. They shared a room, they shared a tragedy and they shared a wild youth and coming of age.
Thom is just catatonic with grief. His world has stopped."