Yes she is - you dont have t be a member of the BRF to be a patron.
Here is the full list from Wiki. The first one is the charity mentioned in tbhe post above.
1990, The Duchess became patron of The Teenage Cancer Trust and has since opened most of the charities various units, including those at Middlesex Hospital, University College London, St James’s University Hospital, Cardiff University Hospital and Royal Marsden Hospital
1993, The Duchess founded
Children in Crisis a children’s charity focused on education and grant making to international programs. The Duchess serves as Founder and Life President.
2003, The Duchess joined the
American Cancer Society at a congressional briefing. Sarah, Duchess of York, was a founding supporter of The American Cancer Society’s Great American Weigh In, an annual campaign (modelled after the Society’s Great American Smoke Out) aimed at raising awareness of the link between excess weight and cancer.
2006, The Duchess established
The Sarah Ferguson Foundation based in Toronto, which derives funds from Sarah's commercial work and private donations with the aim of supporting charities internationally that serve children and families in dire need. Included under this umbrella organisation is her patronage of several British charities, including
Mental Disability Rights International, the
Teenage Cancer Trust,
[55] Tommy's, and the
Motor Neurone Disease Association
2008, The Duchess became patron of Humanitas, a charity focused on providing children with education, healthcare and family support
2010, The Duchess became a supporter of The Mullany Fund, whose aim is to support UK students wishing to study medicine or physiotherapy.
2011, The Duchess became the global ambassador for Not For Sale, a charity focused on human slavery