Johann's sister Margarete Berent (09.07.1887-23.06.1965) lived with them in Berlin and survived by emigrating to NY in 1940. She was the first woman attorney in Prussia. See: wikipedia-Margarete Berent
In addition, a Jokos file (number 8777) on this family is at the Amsterdam Municipal Archive. Access is subject to authorization from the Stichting Joods Maatschappelijk Werk.