That makes sense with the "George" Quin named as father in Ethel's marriage to Charlie Richardson, who - as we've noted before - is a General Fitter (engineering) on the 1939 Register. They are her maternal grandfather, and step grandmother.
In 1939 when Ethel was only 11, she does not appear with William George and his wife Florence (Barrow) Quin, at 15 Earl Street, Salford. With the couple is James Henry Lowe (born 1914, so 25 years old), Florence's son from her first marriage.
I've not been able to find an entry for Ethel in the 1939 with any of the family (including Florence Barrow's relatives), or a redacted entry which might be her.