Betje Walvis was a daughter of Zacharias Walvis and Fijtje Posener. She married on 18 February in Amsterdam to Gerrit Koster, son of Jesaijas Moses Koster and Rosette Bos.
City Archive of Amsterdam, archivecard of Betje Walvis and website www.wiewaswie.
In addition, a Jokos file (number 25059) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.