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

Mary Sinclair is the sister of Horatio and runs the Sinclair household in Macao. She has lived her whole life in the Orient and has never been to England. She speaks two Chinese dialects and is a source of information for Dirk. Mary is not wealthy so Dirk helps her out at times. He tells her of her unknown Aunt Wilhelmina who died in Holland and left her money as a way of seeing she had money for a ball gown and other things for the information she provided him with. She is trusted by May-may. Mary and her brother Horatio have had an incestuous relationship in the past. Mary is pregnant with a Chinese baby when she agrees to marry Glessing. Mary takes Chinese herbs to cause an abortion and is hospitalized in Macao.