Muriel G. Nix contributed to Swift Annual 5 (1958). Other than a short (48-page) collection of stories published in 1939 (listed below), I can find very little about her. She may be the Muriel G. Nix listed in the 1881 England census: Muriel Grace Nix was born in Eastbourne, Sussex, in 1880, the daughter of John (a jeweller) and Mary D. Nix, and was educated at a boarding school in Camberwell. If that's her, she'd have been in her late seventies when the Swift Annual story appeared... but I can't find any trace of another Muriel G. Nix (Muriel Nix being a very uncommon name even without the middle initial).
Mrs. Winkle, and other stories. London, A. H. Stockwell, 1939.