Link: Daily Mail
Most of the engagements that Catherine will be postponing while she recovers are visits to her patronages. They don't want another royal showing up in her place, somebody who is unfamiliar with their organization, not interested in their cause, or worst-case scenario, opposed to their work.
Overseas engagements done at the request of the Foreign Office cannot be given to Beatrice or Eugenie. Seriously, what foreign government is going to agree to "Catherine and William can't make it, but not to worry, we'll send Eugenie and Jack. What are they? She's Andrew's daughter and Jack's her husband."
Catherine's charities have multiple patrons, If there's something they need now, one of their non-royal patrons will see to it.
Maybe Beatrice and Eugenie wouldn't be box office poison if the media didn't feel the need to trash them at every opportunity or feel the need to remind the public for their parents' shortcomings at every opportunity. (This recent headline being an example: Princess Eugenie speaks out against human trafficking at Davos event days after father Prince Andrew was axed as working royal over claims he slept with Epstein trafficking victim Virginia Giuffre https://www.dailymail.co.uk/news/article-12971865/Princess-Eugenie-anti-trafficking-event-Prince-Andrew-Virginia-Giuffre.html)
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