Abraham Koster was a son of Simon Koster, and Bilha Sarphatie. He married on 23 November 1924 to Henriette Con. Municipal Archive of Amsterdam, archive card of Simon Koster.
In addition, a Jokos file (number 11707) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.