Esther Koekoek was a daughter of Abraham Koekoek and Anna van de Star. She married in Amsterdam on 4 June 1930 to Marcus van Praag. City Archive of Amsterdam, archive card of Abraham Koekoek.
In addition, a Jokos file (number 17954) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.