Jozef de Groot was a son of Mozes de Groot and Sientje Nieweg. In that family there were four other children: Samuel, Louis, Marcus and Estella. Jozef married on 8 December 1924 in Groningen to Reintje de Lange. The couple had two children: Sonja and Salli Maurits.
website www.allegroningers.nl, index of marriages.