Cecil C. P. Lawson was JW's nephew by marriage. He was the son of Beatrix Whistler's older sister Constance and the artist Cecil Gordon Lawson.
An oil portrait of him by Beatrix Whistler is in the Hunterian Art Gallery, University of Glasgow.
Later in life he was a military historian and wrote books on military uniforms.
UK census 1881, from http://www.familysearch.org (accessed 2004).