Jan Bernardus Koster, born circa 1789 (?).

Jan was married to
Klaske Jans Groeneveld, born circa 1793 (?).

• 1 : Ernst Jans Koster, born in Franeker on August 1, 1816, died in Harlingen on April 10, 1862, aged 45 years.
  Ernst was married in Franeker on May 14, 1837, at the age of 20 to Trijntje Willems Geldersma (aged 24 years), born in Franeker on September 5, 1812, daughter of Willem Pieters Geldersma and Aafke Pieters Vlietstra, died in Harlingen on January 25, 1902, aged 89 years.