Complex sentence: Kenneth was the third of four sons--eldest was William (born 1893); he emigrated to his mother's family in New Zealand but was killed at the age of 22 in 1915 whilst fighting with the ANZACS at Gallipoli.
Split into simpler sentences: Kenneth was the third of four sons--eldest was William (born 1893) who emigrated to be with his mother's family in New Zealand. He was killed at the age of 22 in 1915 whilst fighting with the ANZACS at Gallipoli.