W B Rhodes' first marriage was in May 1852 to Sarah King, daughter of solicitor John King. She died in August 1862 aged 28. There were no surviving children. In October 1869 after completing The Grange, a new homestead for his Highland Park property, he married Sarah Ann Moorhouse (c. 1837–1914), a sister of William Sefton Moorhouse and Lucy Ellen Sykes Moorhouse who married John Studholme. Though he had no children by either wife, about 1850 he and a Māori woman called Mary had a daughter, Mary Ann. William and Sarah loved his daughter, and Sarah adopted her. She was provided for in her father's will. In spite of this Mary Ann challenged the will. She lost her case in the New Zealand Supreme Court but she was awarded £750,000 on appeal to the Privy Council.