Nicholas Bennett Dalby (b.1826) was the youngest son of Augustus Dalby. He was a major in the Royal Marines. In 1861 he was a captain, stationed at the Royal Marine Barracks, Plymouth Division. He married Helen Lyttleton Bennett in Hove, Sussex, the same year, and they had a daughter Nelly in 1862. I have no information on other children. I also haven't found the family in the 1871 census so he could well have been serving overseas - and with a 20-years gap to 1881 could have had children who stayed in India. Nicholas and Helen were in Bishops Stortford, Herts, in 1881, and Nelly was an assistant teacher at St Margarets Naval School, Isleworth, Middlesex. Of course Ethel Augusta would have been a granddaughter if she was related to Nicholas.
His elder brother William Bennett Dalby was a naval surgeon. He died in 1884 but had at least four sons. I haven't researched Francis Higgs Dalby born 1853 in Penzance, but he seems to have settled in Lincolnshire, from the census entries. The second son, Augustus William, became a GP (born 1859), practising in Frome, Somerset in 1891 and 1901 (though conceivable that he went to India and returned in between). William Edward was born in Devon in 1865, was living there in 1881, and in 1901 was in Leamington, Warwickshire. I haven't found him in the 1891 census. However, he had 4 children in Leamington during the 1990s so I think can be eliminated. Herbert Ernest, born 1868, was with his brother Augustus in Frome in 1891 (living on 'own means'). He married in Christchurch, Hampshire, in 1896, but I lose track of him after that. As it is a military family, he is a possible candidate.