Complex sentence: Andrew Ciechanowiecki was born in Warsaw, the descendant of a Polish noble, well-connected, senatorial family, settled in Belarus, which lost its estates as a result of the Treaty of Riga, when they remained on the Soviet side of the border.
Split into simpler sentences: Andrew Ciechanowiecki was born in Warsaw, the descendant of a Polish noble, well-connected, senatorial family, which had settled in Belarus. The family lost its estates as a result of the Treaty of Riga, when the estates remained on the Soviet side of the border.