What was the name of Hemingway's only brother?

Hemingway’s only brother was Leicester Hemingway. Born in 1915, Leicester was the youngest child in the Hemingway family and Ernest’s younger sibling by about sixteen years.

The brothers grew up in Oak Park, Illinois, with three sisters: Marcelline, Ursula, and Madelaine, plus their younger sister Carol. Unlike Ernest, Leicester was born after his brother had already begun establishing himself as a writer, but he later pursued a literary career of his own.

Leicester wrote six books, including the novel The Sound of the Trumpet, published in 1953. In 1961, he published My Brother, Ernest Hemingway, a biography that brought him recognition and substantial financial rewards.

Leicester also created New Atlantis, a micronation founded on a raft off Jamaica in 1964. He intended it to support marine research and issued its own coins and stamps. A tropical storm destroyed the project in 1966. Leicester died in 1982.