The Betty Quilter who married Keith George Joslin in 1942 was Betty Louisa Quilter, daughter of Richard Quilter and Edith Emily Francis. There's a newspaper item about the wedding, giving Betty's family details and the family's address. It looks like this Betty died in 2007 and husband Keith died in 1992.
Coming back to the Betty born in 1930, there's a newspaper item about her mother's death. Her father was a witness at the Coroner's inquest, and stated he and his late wife had 5 children, the oldest a girl of 17. Which all tallies with the info already posted. The Coroner did question what was to happen to the oldest child, as she had a school scholarship and had just passed her Oxford exam, but no mention of the others.
Newspaper reference:
Leicester Evening Mail
17 Feb 1932
page 7, column 7
If you want a copy of the item let me know, and I'll sort something via PM.