Who is Horatio Sinclair from Tai-Pan and what is their importance?

Horatio Sinclair was an aide to William Longstaff. He could read and write Chinese. His and Mary's father had been one of the first English missionaries in the Orient and had settled in Macao. Horatio was educated in England and then returned to the Orient. He has had an incestuous relationship with his sister Mary and is very angry and bitter when she accepts Glessing's proposal of marriage. Horatio is killed during the typhoon.