FNM's twin brother Henry Edward Campbell Miller (11/3/1859 - 20/1/1949) married Ada Percival Nicholls on 11/8/1880 in Peterbrough Cathedral. According to vaious census returns he was in 1881 a Wine Merchant, in 1891 a Brewery Manager, 1901 not found, and in 1911 an Insurance Manager (presumably in his brother's firm). Henry had three daughters, Eva Percival Miller (b. 1881), Gladys Ada Gay Miller (b. 1882) andf Catherine May Miller (b. 1/5/1884). I know nothing further about Eva but the other two married and had children.
As far as I know FNM had only 2 brothers (Henry and Alfred Ernest (b. Feb. 1861)) and 2 sisters (Jessie Ann (1857 - 1874) and Lilian Helen (1866 - 1892)).
If anything further comes to mind I will post it in due course.