Hi
Thanks for replying. The story is a bit complicated.
According to their marriage certificate, my grandfather John Sinclair Holmes, 23, bachelor, married Jessie Margaret Nicholson, 20, spinster, in Glasgow in 1896. They had 2 sons that I know of: James Holmes, 1897-1975 (he later changed his name to James Holmes Nicholson, using his mother's maiden surname as his surname), and John Nicholson Holmes, born 1899, who died in 1900.
At some point my grandfather John and Jessie separated. According to both the 1901 and 1911 census records, Jessie was living with her parents and son James. John was not there, and there is no mention of Robert, born about 1908, in the 1911 census.
John divorced Jessie in 1919, just months before marrying my grandmother. Jessie didn't remarry and died in 1925.
So far, everything makes sense. However, the informant on James's 1975 death certificate is "Robert Nicolson, brother". I've been unable to verify this relationship (which would make him my father's half-brother) as I can't find a birth or death certificate for Robert, and only have a copy of a Minor Record of his death in Majorca. This doesn't give parents or next of kin, and the informant is a doctor at the clinic where he died.