Machiël Koster was the son of Izaak Koster and Judit (Jetta) van Engers. He married Duifje Ossendrijver on 19 August 1896. She was the daughter of Eliazar Ossendrijver and Engeltje Poons. The couple had four children, among whom Engeltje.
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In addition, a Jokos file (number 5296) on this household is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.The Jokos file reveals that a claim was lodged for compensation for valuables surrendered to the Lippmann-Rosenthal looting bank (L-claim, number 6721/16222).