Her parents were Elizabeth (born Macarthur) and Captain Arthur Alexander Walton Onslow, R.N.

[1] While her brothers were sent to private schools in Australia and Britain for their education, she was taught by a German tutor and her mother.

Sibella would organise frequent musical evenings at Camden Park which were led by Emmeline Woolley and Ethel Charlotte Pedley.

[3] Her mother's father was an important figure as he had helped to establish the Australian wool industry.

[6] A reception in her honour attracted many VIPs including NSW Premier Thomas Bavin.