Herbert's wife Cathleen Synge Morawetz was a prolific mathematician at NYU. His sister Sonja Morawetz Sinclair revealed in 2017 she was a WW2 codebreaker after seven decades of secrecy by Bletchley Park Signals Intelligence. He helped organize the defection of Mikhail Barishnikov from the USSR 1974.  His brother, Oskar Morawetz was a Canadian composer Oskar Morawetz.  His brother John Morawetz was a Canadian businessman.