The person that is the mother of XXX is who?
Friedrich Kettler
Friedrich Kettler (Latvian: Frīdrihs Ketlers, 25 November 1569 in Mītava (German Mitau, now Jelgava) -- 17 August 1642) was Duke of Courland (Latvian: Kurzeme, now part of Latvia) from 1587 to 1642.