How do McEwan and Sharp share the same parents if their mother was married to a different man when Sharp was born?

Context: In 2002, McEwan discovered that he had a brother who had been given up for adoption during the Second World War; the story became public in 2007. The brother, a bricklayer named David Sharp, was born six years earlier than McEwan, when their mother was married to a different man. Sharp has the same mother and father as McEwan but was born from an affair that occurred before they married. After her first husband was killed in combat, McEwan's mother married her lover, and Ian was born a few years later. The brothers are in regular contact and McEwan has written a foreword to Sharp's memoir.

Answer: While married to a man who would die in combat, their mother had an affair with a man (their father) she would later marry. That affair produced Sharp, and they proceeded to have another child, McEwan, following their marriage.