From the small monochrome photograph he attached, it appears to me that the subject is in fact Sybil’s mother-in-law, Lady Mary Nicholson, nee Romilly (1841-1921), and that the artist was Louisa Canziani Starr [British, 1845-1909].
If so, the painting was exhibited at the Royal Academy in 1884, The New Gallery in 1892 and The United Arts Club in 1908.
Confusingly, Herbert Arnould Olivier [British, 1861-1952] painted his sister, another, distantly related, Lady Nicholson (Lady Evelyn Louise Nicholson, nee Olivier) which was exhibited at the RA in 1894. Although the photograph seems more like a Canziani than an Olivier. I have been unable to find either painting’s whereabouts now, nor any mention of a portrait of Lady Sybil Nicholson.
I’d get out more, but I keep going down rabbit holes.
The Maas Gallery
Duke Street, St James’s