The Kennedy fortune was made by Joseph Kennedy, the father of John and Teddy. Joseph Kennedy was the front man for J.P. Morgan, the banker, who also bankrolled Thomas Edison. Kennedy would start rumors on stock hikes for stocks Morgan owned a lot of, then they would sell high. He would also bad mouth stocks that Morgan wanted to buy, driving the stocks up. He became known as the 'Wolf of Wall Street'. The original. Much of this sort of manipulation became illegal in the late 1920's and early 1930's. Joseph Kennedy always operated just inside the law.
When Prohibition was repealed, Kennedy was the US Ambassador to Great Britain. He got that job to get him out of Wall Street. It is rumored that he was responsible for the crash