I was told that Alida Barendina Kruger was the niece of @President Stephanus Johannes Paulus Kruger, and that her father was called Johannes.
The President had two brothers who were called Johannes: @Theunis Johannes Kruger and a half-brother @Gert Johannes Kruger. I cannot find evidence that either had a daughter called Alida Barendina, who then married FN Viljoen. I also cannot find details about Alida's mother. I was told she was called Catharina Maria, but the surname is unknown.
If you have any information about this, please let me know!