Marais grew up on the farm Wonderfontein close to Robertson, Cape Colony. As a child in school his brother Eksteen (Johannes Eksteen) and he had a workshop and laboratory on the farm. In the workshop they generated electricity. In the laboratory they experiment with gunpowder. In 1921 Kosie had to stop his studies at Stellenbosch University and had to farm as his father Kowie was ill. His father died in 1922. Eksteen and Kosie became owners of the farms Klipdrift and Wonderfontein. His sister Judith Maria Magdalena, helped on the farm too. She later married a de Wet. His oldest brother, Ernst Jacobus, died young in 1918. Kosie operated on Klipdrift and Eksteen on Wonderfontein. On Klipdrift Kosie made brandy and liqueurs. In 1957 the two brothers split. Kosie got Klipdrift, and Eksteen Wonderfontein. Kosie was a South African Freemason.
