Seen here with his father, a rare creature known as "Trump With Empathy."
Let's back up. Donald Trump had a big brother who died at the young age of 43 in 1981. Freddy Trump wasn't as into real estate as his little brother or dad, and by the time he died, he had a reputation as an alcoholic, which some people think is why Donald himself is a vigorous teetotaler -- he watched his big brother destroy himself with booze. Before Freddy died, he got married and had two kids, Mary and Fred III. Those two kids grew up, got married (not to each other), started their own families, and moved on with life.
Then something weird happened. Somewhere along the way, Donald Trump's father decided to exclude his dead alcoholic son's heirs from his will. So when Fred Trump Sr. died in 1999, the millions of dollars he left behind were divided among Donald and his two living siblings -- everyone in Freddy's family was cut out of the inheritance. Specifically, Donald Trump helped his father draft a will which divided "at least $20 million, among his children and their descendants, 'other than my son Fred C. Trump Jr.'"
To be fair, if you were forced into an old-man combover since your 20s, you'd lash out at others too.