Complex sentence: He was the younger brother of Edith Sitwell and Osbert Sitwell; later in life he became more reticent about associating himself with the publicity attaching to the Sitwells collectively, travelling and concentrating on writing.
Split into simpler sentences: He was the younger brother of Edith Sitwell and Osbert Sitwell. In his later life he became more reticent about associating himself with the publicity attaching to the Sitwells collectively, travelling and concentrating on writing.