Answer by taking a quote from the following article:

Robert Ervin Howard (January 22, 1906 - June 11, 1936) was an American author who wrote pulp fiction in a diverse range of genres. He is well known for his character Conan the Barbarian and is regarded as the father of the sword and sorcery subgenre. Howard was born and raised in Texas. He spent most of his life in the town of Cross Plains, with some time spent in nearby Brownwood.

Howard was born January 22, 1906 in Peaster, Texas, the only son of a traveling country physician, Dr. Isaac Mordecai Howard, and his wife, Hester Jane Ervin Howard. His early life was spent wandering through a variety of Texas cowtowns and boomtowns: Dark Valley (1906), Seminole (1908), Bronte (1909), Poteet (1910), Oran (1912), Wichita Falls (1913), Bagwell (1913), Cross Cut (1915), and Burkett (1917).  During Howard's youth his parents' relationship began to break down. The Howard family had problems with money which may have been exacerbated by Isaac Howard investing in get-rich-quick schemes. Hester Howard, meanwhile, came to believe that she had married below herself. Soon the pair were actively fighting. Hester did not want Isaac to have anything to do with their son. She had a particularly strong influence on her son's intellectual growth. She had spent her early years helping a variety of sick relatives, contracting tuberculosis in the process. She instilled in her son a deep love of poetry and literature, recited verse daily and supported him unceasingly in his efforts to write.  Other experiences would later seep into his prose. Although he loved reading and learning, he found school to be confining and began to hate having anyone in authority over him. Experiences watching and confronting bullies revealed the omnipresence of evil and enemies in the world, and taught him the value of physical strength and violence. Being the son of the local doctor gave Howard frequent exposure to the effects of injury and violence, due to accidents on farms and oil fields combined with the massive increase in crime that came with the oil boom. Firsthand tales of gunfights, lynchings, feuds, and Indian raids developed his distinctly Texan, hardboiled outlook on the world. Sports, especially boxing, became a passionate preoccupation. At the time, boxing was the most popular sport in the country, with a cultural influence far in excess of what it is today. James J. Jeffries, Jack Johnson, Bob Fitzsimmons, and later Jack Dempsey were the names that inspired during those years, and he grew up a lover of all contests of violent, masculine struggle.

Did he have any siblings?
only son