Complex sentence: Hennerici had a twin brother Heinz who lost his left arm in the Second World War, however was a devoted racing driver who still holds a record in his group at the Nürburgring.
Split into simpler sentences: Hennerici had a twin brother Heinz who lost his left arm in the Second World War. However, he was a devoted racing driver who still holds a record in his group at the Nürburgring.