STOLTZ FAMILY FROM BENONI

Good day
My grandfather, Abel Stoltz moved from Middelburg to Benoni in 1930. He second wife was Sophie. How can I find more info before and during that time? My father was Gerrit Coenraad Frederik Stoltz. Brother's: "Daniel also called Daantjie", "Andries", "Mickey" and sister whom they called "Sussie". I don't recall any other brothers.