Your parents' marriage record may have her maiden name. His birth or death record should also have it. Were they buried in the same place? Surrounding graves might indicate a relationship. I'm not familiar with South African records, so I won't be of much help.
As your father's daughter you should be able to request these records from the government, if they are not publicly available.
Another source might be obituaries, which often name parents--check those for your father and his mother and father.