The mother of XXX is whom?
Siemowit I of Masovia
Siemowit I of Masovia (Polish: Siemowit (Ziemowit) I mazowiecki) (c. 1224/28 -- 23 June 1262), was a Polish prince member of the House of Piast, Duke of Czersk during 1247-1248, Duke of Masovia (except Dobrzyń) during 1248-1262, ruler over Sieradz during 1259-1260 He was the sixth son of Konrad I of Masovia and his wife Agafia of Rus.
Agafia of Rus