Trump's abstinence from alcohol was largely shaped by the death of his brother, Fred Jr., from alcoholism in 1981. He was just 42 years old.
In a rare glimpse at the softer side of Trump, the billionaire businessman tells PEOPLE that his brother's death had a "tremendous impact" on his life.
"He was a great guy, a handsome person. He was the life of the party. He was a fantastic guy, but he got stuck on alcohol," says Trump, 69. "And it had a profound impact and ultimately [he] became an alcoholic and died of alcoholism. He would tell me, 'Don't drink ever' … He understood the problem that he had and that it was a very hard problem."
"He had a profound impact on my life, because you never know where you're going to end up," Trump adds. "I've known so many people that were so strong and so powerful [yet] they were unable to stop drinking."
The father of five – and grandfather of seven (with another on the way) – tells PEOPLE he also impressed the importance of abstaining from drugs and alcohol on his children. "I think they drink a little bit, but not much. But I say no drugs, no alcohol, no cigarettes."
Trump told PEOPLE for his cover story last week that his family has always been the most important thing in his life.
"I always prided myself on being a good father," he said. "With my children, I was always available."
His daughter Ivanka, 33, agreed: "He was tough, firm, but always available to us."
Reporting by JESS CAGLE and CHARLOTTE TRIGGS