Jacob Loonstein was a son of Emanuel Jacob Loonstein and Saartje Boutelje. He married in Amsterdam on 7 September 1892 to Naatje Speijer, daughter of Jacob Leendert Speijer and Marianna Emanuel Schrijver. Jacob and Naatje had 9 children: Emanuel, Marianne, Maurits Jacob, Herman, Isidore, Simon, Sara…
In addition, a Jokos file (number 55885) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.