What Duchy founded by the Lithuanians, was Kestutis the Grand Duck of? 
Birutė: Birutė (died 1382) was the second wife of Kęstutis, Grand Duke of Lithuania, and mother of Vytautas the Great. Grand Duchy of Lithuania: The Grand Duchy of Lithuania was a European state from the 13th century until 1795. Grand Duchy of Lithuania:  The state was founded by the Lithuanians, one of the polytheistic Baltic tribes from Aukštaitija.
Grand Duchy of Lithuania