This is a long shot, but the mother's name could certainly be abbreviated to Sanni or something similar.
South Africa, Netherdutch Reformed Church Registers (Pretoria Archive)
Name Catharina Maria Rheeder
Event Type Baptism
Event Date 21 Oct 1906
Event Place Lichtenburg, Transvaal, South Africa
Birth Date 13 Jul 1906
Father's Name Jacob Salomon Rheeder
Mother's Name Gysiena Wilhelmina Swart
There is a brother, I wonder if this is the Johanna you mention or was that definitely a girl?
Name Jan Johannes Jacobus Rheeders
Event Type Baptism
Event Date 9 Jul 1911
Event Place Lichtenburg, Transvaal, South Africa
Birth Date 15 Apr 1911
Father's Name Jacob Samuel Rheeders
Mother's Name Gezina Wilhelmina Swart
Bev