The Duchess of York has spoken of her pride in daughters Beatrice and Eugenie as she made the plea: "Stop bullying the York family."
She was speaking as her daughters became patrons of the Teenage Cancer Trust, calling their "hard work" an example of "good parenting".
The Duchess and the two princesses visited a specialist teen cancer unit in London to mark the occasion, meeting young people with the disease.
Sarah said: "Both the Duke and myself, we could not be more proud of this moment because these are two girls that work so hard in their own careers, have taken time off today to spread the word of teen cancer, which is so important, and then go back to work.
"It's really an extraordinary example that, I think, good parenting and listening to your children and getting them to take responsibility for their own actions."
The Teenage Cancer Trust provides specialist care and support for young people with cancer, giving them the chance to connect with others of the same age.
Beatrice spent time talking to Harry Sadler at the University College Hospital unit, who has been diagnosed with a rare form of brain cancer.
The 17-year-old said the princess was polite and "vibrant", and he added: "She listened to me."
Beatrice was in turn inspired by the Colchester student, saying: "His strength taught me so much. I learnt a huge lesson today from Harry."
Mr Sadler said he likes to think positively, and does not listen to everything doctors say. "I'm a very strong believer in mind over matter," he said. "Statistics are just numbers. Humans are so much more than numbers."
Patient Elias Taylor, 20, said of the visit: "I didn't feel like I was talking to royalty. It's nice because they're quite similar in age to me."
The student relapsed after originally being diagnosed with cancer aged 16, and has been undergoing treatment for acute lymphoblastic leukaemia. He has taken a break from his business management degree in Birmingham to have the treatment.
The unit where Mr Taylor has been staying is designed for young people, with a living room, gaming equipment and a jukebox.
He joked: "I would love to meet a couple of Arsenal players but I'll take royalty."
Teenage Cancer Trust chief Siobhan Dunn said it is crucial for the charity to raise more funds.
"Right now, for every young person we reach there's another we can't, but we are determined to change that," she said.
"Sarah, Duchess of York, has been the most extraordinary supporter of Teenage Cancer Trust over the years.
"She has been with us from day one and we're so grateful for her introducing Princess Beatrice and Princess Eugenie to the charity."
Sarah said she is proud of her family's charity work, and added: "Let's focus more on this and less on tittle-tattle gossip.
"Stop bullying the York family, please."
The princesses have faced criticism in the media over their public roles.
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