According to reports, 19-year-old Stuart Kelly, the younger brother of Thomas, died in Sydneyโs Mona Vale on Monday.
The death is not being treated as suspicious and a report is being prepared for the coroner.
Kingโs School headmaster Dr Tim Hawkes confirmed the heartbreaking news in a statement which read, โIt is with great sadness that I inform you of the death of an Old Boy of the School, Stuart Kelly (โ15).โ
โThis is the second great tragedy to affect the Kelly family.โ
โThe exact circumstances surrounding his death are not known, however, it is enough to know that we have lost a member of our community and therefore our thoughts and prayers go out to Kathy and Ralph Kelly and their daughter Madeleine.โ
โAs a student, Stuart was greatly respected, He was a school prefect who possessed a quiet integrity that made him enormously effective as a leader.โ
โThe strength and character shown by Stuart when his brother was killed by a one punch attack in July 2012, was extraordinary,โ the statement concluded.
In the aftermath of his older brother Thomasโ senseless death, Stuart often addressed the media to raise awareness of their story.

Thomas Kelly, Stuartโs older brother, died in 2012 from a one-punch attack.
โI believe we should have respect for one another, for our friends, family, acquaintances, different cultures and complete strangers,โ Stuart said after Kieran Loveridge was sentenced to at least 10 years jail for manslaughter.
โA culture where we accept responsibility for our actions โ not one where we all too often lay the blame on our past as an excuse for what we do today.โ
He also worked closely with The Thomas Kelly Youth Foundation, which was set up by the family to help campaign for โbehavioural change and a push towards changing Australiaโs drinking culture through education.โ
Watch Stuart talk about the harrowing loss of his brother in the video below. Post continues afterโฆ
Last year, Stuart moved the crowds at a fundraising dinner for the charity with a powerful speech (above).
โI look back at that moment: I was 14 years old, I was told by a stranger that my brother, my best friend, was going to die. Those few words would change my life forever,โ he explained in the stirring delivery.
โIโm now 17 โ that was three years ago. However I carry a deep scar that you cannot see. Itโs always there, it never leaves. It sits below the surface of your skin and surfaces when you least expect it.โ
โโWe are not alone, there are many many thousands of other who are directly affected by senseless violence every year. Premier will you make this promise tonight? Australia is an alcoholic, we need to rethink the way we drink,โ Stuart said.
Our thoughts are with the Kelly family at this difficult time.

Last year, Stuart Kelly delivered a moving speech about his brotherโs untimely death while calling out the nation for its binge drinking culture.