His father was Johann Georg , elector of Brandenburg, born 1525 ,married to Sophie von Liegnitz.
Joachim Friedrich's sister was Sophie of Brandenburg Hohenzollern, born 1568. Sophie married [Christian I]? Wettin, elector of Saxony,son of August Wettin, elector of Saxony. From that line came the later Frederick [August I the Strong]?, Wettin, elector of Saxony and king of Poland .