Complex sentence: His granduncle was Bedřich Wachsmann and his cousin was Alois Wachsman, both Czech painters and architects, while his uncle was Austrian painter Julius Wachsmann (1866--1936).
Split into simpler sentences: His granduncle was Bedřich Wachsmann and his cousin was Alois Wachsman, both Czech painters and architects. Also his uncle was Austrian painter Julius Wachsmann (1866-1936).