Ok, so I noticed something else... here's the full inscription from Toxteth Cemetery:
In loving memory of / our dear mother / Elizabeth BENN, / died 6th Jan. 1927, aged 72 years. / Also Ernest BENN, / who was lost at sea 20th March 1918, / aged 28 years. / Also Margaret BENN, / died 26th Dec. 1911, aged 28 years. / *Peace, perfect peace.* / In loving memory of / my dear husband / Harry BENN, / died 18th April 1931, aged 44 years. / *He giveth His beloved sleep.*
Checking up BMD records I found that Harry Benn was actually christened Henry Benn. He was married in 1910, became a widower in 1911 (Margaret on the headstone) and remarried in 1918 (signing both Harry and Henry on the certificate) then died in 1931.
So, simplest answer seems to be that both brothers were in the Mercantile Marine and there was a mix up when Ernest died on the Olympic.
Any thoughts?