The brothers' relationship is documented in this reference, which states:
East Suffolk Gazette 7 Dec 1925: DEATH of Colonel McComb, youngest brother of Dr WT McComb, late of Beccles. Commissioned in Army Service Corps in 1874. Service in Egyptian War 1882 (medal & Kedive’s Star); Bechuanaland Expedition 1884-85, Zulu War 1888, Boer War 1899-1902. (medals). He served throughout the Great War.
This is almost certainly Colonel R. B. McComb, whose military career is documented elsewhere, including:
Who's Who, which gives his dates as: Born 22 Nov. 1855; died 18 Nov. 1925
London Times - 5th October 1899, p5:
A list of the officers and troops who leave Southampton for SA [Boer War] to-morrow in the Braemar Castle: [including] Army Service Corps. - Col. R.B. McComb.
The Times, Saturday, Oct 25, 1902; pg. 8:
Investiture, Order of the Bath (C.B.)
Colonel Robert Brophy McComb, the Army Service Corps