Complex sentence: On 21 March 1883 he married Mary Cameron Murray, but she died five months later; on 24 December 1884 he married her older sister Isabella, with whom he had three surviving children.
Split into simpler sentences: On 21 March 1883 he married Mary Cameron Murray, but she died only five months later. On 24 December 1884, Parkes married her older sister Isabella, with whom he had three surviving children.