Sibylle Cole-Duijts was a highly accomplished painter, fl. 1950's - 1990's.
She was born and brought up in Amsterdam but lived most of her adult life in the UK.
She trained at Birmingham School of Art in the 1940's under Bernard Fleetwood-Walker, Harold Smith etc and, possibly, at one of the London schools.
She painted mainly in oils and water-colour but was also skilled in other media, particularly pencil-drawing.
She exhibited widely throughout her life and had work shown at the RA (she had several works accepted and hung there) the Royal Birmingham Society of Artists, the Yorkshire Artists, Ferens Gallery in Hull and elsewhere. She also had a number of one-woman shows.
She was one of that group of artists that came to prominence in Birmingham in the 1950's/1960's (others in this group included Gilbert Mason, Katherine Fryer, Ruth Burden, David Prentice and John Bridgeman)
Her work is in many collections, both private and public, throughout the world. Unfortunately, her work rarely comes to the market which possibly accounts for the fact that she is not better known.
She signed her paintings variously "Sibylle Cole-Duijts" "Sibylle Cole" or "Sibylle Duijts", sometimes abbreviating her first name to the inital only. Her paintings are, however, not always signed.
You are right about the Fred Elwell connection. In 1975, at a time when Elwell was little known beyond Beverly, she had published "Frederick W Elwell RA, ROI 1870-1958 - A monograph by Sibylle Cole"
It may be worth approaching Beverley Art Gallery and/or the RBSA. If they cannot help you they should be able to point you in the right direction.