Mr Sinclair was the father of Albert, (AUDIO: UNIT Dating) Harry, Helen and George Sinclair. He was cruel and bullied his wife and daughter whilst treating his sons adequately due to their gender. (AUDIO: The Red Lady, Absent Friends) He disowned Albert when he was caught with another man and sent to prison, and tore up all photographs of his son, turning angry at the mere mention of his name. (AUDIO: UNIT Dating)
In World War II, Mr Sinclair was conscripted and sent to war. According to Helen, this was his first time fighting in uniform. Helen and her mother's relationship greatly improved as a result of his absence, which allowed them to grow closer. Helen later characterised her father's conscription as "the best thing that ever happened to [them]". (AUDIO: Baker Street Irregulars)
In 1963, Helen was accused of stealing from the National Museum and disappeared, severely damaging the Sinclairs' reputation and, in Helen's mind, proving her father right about her. He died a few years apart from his wife, driven to an early grave by Helen, according to George. (AUDIO: Absent Friends)