This is the wedding register for Maria Elizabeth Viljoen and Hendrik Lambertus Bronkhorst
https://www.familysearch.org/ark:/61903/3:1:3QS7-99ZR-439H?i=74&...
In the Death Notice of Maria it gives her parents names (written in pencil) Christoffel Viljoen and Beth van Vuuren (Elizabeth Janse van Vuuren)
They had a daughter Maria Elizabeth Viljoen that was the third wife of Hermanus Gerhardus Grove. There were numerous children from the marriages ages not verified but can I pose the theory that some of the 20 children listed on the Death notice includes step children?
I havent found or seen a list of the names of all the children (yet, Im sure in time I will hopefully find something or someone will share it) but it could be one theory that they arent all Bronkhorst Born children.
Sometimes a lot of what we do is..Best Guess until proven right or wrong.