Walter did have a brother 'Thomas', a chemist. He is alive and well in Oct 1916 and still a chemist and not in the forces, I know this because he is listed in Walter's intestate succession in his service papers.
this is the list:
Father.......Jas. W. Lamb..............59......Warkworth Acklington Northd
Mother.......Minnie Florence Lamb...51......Warkworth Acklington Northd
Brother.......Henry Lamb..............30......C/O Mr. Hollings, Burnside, Wallsend
Brother.......Thomas .................. 28......Chemist Foresthall N/C
Brother.......Jas. W......................23......Serving in France 2nd Lieut.
Brother.......John........................20...... H.M.S. Roxburgh Home Address
Sister.......Elinor Elisabeth..
[sic].........17......Warkworth
Sister.......Minnie Florence..............15......Warkworth
Brother.......Cyril..........................13... ...Warkworth
Certified by James Fairbrother, vicar of Warkworth and dated 18th October 1916