Alethea Garstin (1894–1978) was a Cornish artist and illustrator; she was the daughter and pupil of painter Norman Garstin.
Biography on Wikipedia: https://en.wikipedia.org/wiki/Alethea_Garstin

The Reverend Charles Francis Benthall on Holiday in Morocco by Alethea Garstin
c.1925 / Oil on canvas / 39″x31-1/10″ / Benthall Hall, Shropshire, UK

Alethea Garstin on Artnet: http://www.artnet.com/artists/alethea-garstin/

Further reading:
https://artuk.org/discover/artists/garstin-alethea-18941978
https://www.stephenongpin.com/artist/236757/alethea-garstin
https://cornwallartists.org/cornwall-artists/alethea-garstin