Complex sentence: In 1926, his father, a bookshop owner, whose shop was nationalized by the communists, was sent to a gulag in Karelia while his mother was arrested later the same year and sent to a gulag in Siberia, on the shores of the Yenisey river.
Split into simpler sentences: In 1926, his father, whose bookstore had been nationalized by the communists, was sent to a gulag in Karelia. His his mother was arrested later the same year and sent to a gulag in Siberia, on the shores of the Yenisey river.