Complex sentence: Cornelius Gurlitt's parents were the art dealer Hildebrand Gurlitt from well known family Gurlitt and his wife Helene née Hanke, and he grew up in Dammtor district of Hamburg together with his sister Renate, who was born there in 1935.
Split into simpler sentences: Cornelius Gurlitt's parents were the art dealer Hildebrand Gurlitt from well known Gurlitt family, and his wife Helene née Hanke. He grew up in the Dammtor district of Hamburg with his sister Renate, who was born there in 1935.