In a telephone interview last week, Mr. Trump said he had learned by watching his brother how bad choices could drag down even those who seemed destined to rise. Seeing his brother suffering led him to avoid ever trying alcohol or cigarettes, he said.

But the painful case of Freddy Trump, eight years his brother’s senior and once the heir apparent to their father’s real estate empire, also serves as an example of the dangers of failing to conform in a family dominated by a driven, perfectionist patriarch and an aggressive younger brother.

In the upwardly mobile Trump family, Donald was the second and favorite son, the one who got into the University of Pennsylvania’s Wharton School, relished the combat of New York real estate and ultimately made the Trump name an international brand. Freddy was the disappointment, who lacked the killer instinct and drifted so far from his father’s ambitions that his children were largely cut out of the patriarch’s will.

Freddy, as he was known, “was caught sort of in the middle as somebody who didn’t really love it, and only because he didn’t really love it, he wasn’t particularly good at it,” Mr. Trump said. “My father had great confidence in me, which maybe even put pressure on Fred.”

Asked whether Freddy’s experience in the family business, which friends described as miserable, contributed to the drinking that ultimately killed him, Mr. Trump said: “I hope not. I hope not.”