Sean Euan Lawson, 21, went to bed last Monday and never woke up. His sudden death came 10 years on from that of his sister, Moira, who died at the age of 20.
Doctors say Sean - called Euan by his family - was a victim of Sudden Death Syndrome - the same condition that claimed Moira's life.
His parents, Mariette and Andrew Lawson, and sister Rhona yesterday said they were in "shock" at the second tragedy.
Sean, 21, had hoped to start a career in the RAF - but died at his flat in Dundee's Provost Road last Monday.
His father, Andrew, 60, a lorry driver for Tesco, said: "He had his whole life ahead of him and then he goes to bed one night and that's it, he never wakes up again - it's heartbreaking.
"He had lots of friends, his sense of humour was fantastic and he cared for people."