Found this BISHOP Herbert John born 1916 Male left 1919 from Birkenhead bound for South Africa Cape. To view full details you need to buy credits. I found this at this link
http://www.findmypast.co.uk/passengerListPersonSearchStart.action?redef=0 I typed in his first name surname sex and country South Africa that is all then hit enter. I also found this on the same link BISHOP R V born 1898 Female left 1921 London South Africa Durban (Port Natal. Could this be the sister?
Found this as well DEPOT VAB (Bloemfontein) SOURCE HG TYPE LEER VOLUME_NO 7/5/1/52 SYSTEM 01 REFERENCE 79/1931 PART 1 DESCRIPTION ADMINISTRATION OF JUSTICE. LUNATIC. PATIENT. HELGA EMMERENTIA VLOTMAN. STARTING 19310000 ENDING 19310000 REMARKS BLOEMFONTEIN. At least now we know she landed up in an asylum in Bloemfontein.
Could Herbert have remarried? Found this DEPOT TAB SOURCE MHG TYPE LEER SYSTEM 01 REFERENCE 4729/74 PART 1 DESCRIPTION BISHOP, HERBERT JOHN.STARTING 19740000 ENDING 19740000 REMARKS SURVIVING SPOUSE ALICE JANETTE BISHOP.
I would apply for a copy of Helgas file as it may tell you who her children etcetera were.
To obtain pictures of documents held in the National Archives. Go to
http://www.eggsa.org and select Content --> On Line Shop --> order Archive documents. Once you have selected the currency, select "Enter the Shop" and then select "Archive photos" (in yellow on the blue band) and follow the instructions. They are only able to organise the photographing of documents from the South African Archives at Pretoria (TAB), Bloemfontein (VAB), Pietermaritzburg (NAB) and Durban(TBD). Other archives such as KAB require a researcher to transcribe documents by hand as no copying by camera or photocopying is allowed. Rates are cheap especially if ordering from overseas.
Hope this helps regards Derek.