Sylvia Vera Stevenson was born on 14 March 1936 in Rochester, Kent. She was the first-born daughter of Hubert Mark Stevenson and Vera Mabel Stevenson (née Yeman).

She was closely followed by her sister, Cynthia Mary Goodbrand (née Stevenson) eighteen months later on 8 September 1937, whilst they were living in Brompton Lane, Strood. Sylvia commented that, "She arrived in a hurry and hasn't stopped since!"

She remembered that her Mum, when she was about to take the girls out, would always get Sylvia ready first as she would stay where she was put, whereas Cynthia would undress and escape to the garden to eat worms as soon as Mum's back was turned.

The two sisters had contrasting personalities: Sylvia was usually solemn and serious, whereas Cynthia was fidgety and smiley. Sylvia loved to read a book, although Cynthia was rarely still enough to do so. They loved one another dearly and cared for one another throughout their lives.