Isaäc Soester was a son of Levie Soester and Marianna Sigaar. He married Sientje Buitenkant in Amsterdam on 3 March 1904, a daughter of Arie Buitenkant and Belia Soep. The couple had two daughters: Marianna (she survived the Holocaust) and Belia. Sientje however passed away on 18 March 1918 and was interred on 20 March 1918 in the Jewish Cemetery at Muiderberg.
Isaäc Soester remarried 18 November 1920 in Zaandam Sientje's sister Esther Buitenkant. From this marriage there was no offspring.
City Archive of Amsterdam, archive card of Isaäc Soester, Esther Buitenkant and website www.wiewaswie.nl.