My great grandfather was also Thomas Mitchell born 23/09/1841, his father was Hugh who married Jemima Calder in Dublin 02/07/1840. Thomas the family bible in my possesion states had a sister Jessie born 02/05/1847 just as you stated so I would love to know who you are if you were not born in in South Africa then your grandfather has to Ernest James as Thomas only had 3 children, Hugh (my grandfather) who went to South Africa, Florence who died young followed shortly by her only daughter, leaving only Ernest to have children outside of South Africa