A website called “The Long Long Trail” documents many investigations into aspects of the First World War; of which not the least melancholy is their investigation into how many sets of brothers served in the British Army (including the Commonwealth forces) and who are known to have died on the same day. Currently their answer is 336 (that is 336 sets of brothers), and they are individually documented at https://www.longlongtrail.co.uk/brothers-died-in-the-great-war/
There are many sad stories here: sons who died on the same day while serving in separate units ; twins who died on the same day; even several sets of three brothers who died on the same day.
One particularly took my eye: on 30 June 1916, SD/2706 Pte Cecil Honeysett and his brother SD/2707 Pte James George Honeysett of the 13th Royal Sussex, born in Brightling, Sussex; enlisted Bexhill, Sussex; killed in action during the assault on The Boar’s Head. No doubt these were related to the same Honeysett family who moved from Ashburnham to Brightling in the late 1930s and lived at Hunters Farm until 2009.
The Battle of Boar’s Head took place on the day before the battle of the Somme and was intended as a diversionary tactic. Consequently it has been largely forgotten. So many men died from the 11th, 12th and 13th Southdown battalions of the county’s Royal Sussex Regiment that 30th June 1916 has been described as “The Day Sussex Died”. A book by John Baines with this title has been well-reviewed and is available from Rother Books in Battle.