Miss Rosa Sibella Macarthur-Onslow inherited Camden Park for her lifetime on the death of her mother Elizabeth in 1911. She was born and raised in the house and lived there with her widowed mother. A very active church and charity worker, she concerned herself in many areas of community need. She was educated by her mother and a tutor. She enjoyed singing, dancing and acting. Musical evenings and a choir were organised by musical friends from Sydney. She supported many local activities including the Red Cross Society, St John’s Church of England, St James’ at Menangle and the Australian Board of Missions. Sibella travelled several times to England and Europe. Promoting women’s suffrage through the Peoples’ Reform League of NSW, she represented the National Council of Women (NSW) at the 1927 International Council of Women at Geneva, Switzerland.

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In 1931, aware of her mother’s wishes that Camden Park remain in the family in perpetuity, Sibella exchanged houses with her brother James, moving to nearby Gilbulla, Menangle. Both Camden Park and Gilbulla were used to host garden parties for Red Cross and church fund-raising activities. She received the C.B.E. award for her community services in 1931. Miss Onslow was a much admired member of the pioneer family.