The Duke and Duchess of Sussex said they wish Kate “good health and healing” after she announced her cancer diagnosis.
In a short statement, Prince Harry and Duchess Meghan said they hoped Kate and her family could heal “privately and peacefully.”
Duchess Kate said on Friday that the news was a “huge shock” after an “incredibly difficult few months”.
She said she was “feeling well” and “getting stronger every day.”
Details of the cancer have not been disclosed. Kensington Palace said it was confident the princess would make a full recovery.
Duchess Kate underwent abdominal surgery in January, but it was not known at the time whether she had cancer or not.
Subsequent tests revealed that he had cancer.
Prince Harry and Duchess Meghan stepped down as senior royals in January 2020.
The couple moved to California in June of the same year, wanting more space to raise their son, Archie. Their second child, Lilibet, was born the following year.
King Charles was also recently diagnosed with cancer and flew to London in early February to see his father the day after Prince Harry began treatment. He did not meet his brother during his travels.
Later, in an interview with ABC News, Prince Harry said he was “grateful” to spend a short time with his father in London.
He also agreed that these health concerns can bring families closer together.
On Friday, US President Joe Biden's press secretary Karine Jean-Pierre said the Duchess Kate's news was “terrible” and the White House was praying for the British royal family's full recovery.
“I think it's important to respect their privacy, especially at this time,” she said. “I'm not going to go any further.”
Prince Harry's relationship with his brother has reportedly deteriorated since he announced he was stepping back from royal duties.
In recent years, the brothers have rarely been seen together.
But in 2022, following the Queen's death, Prince William, Duchess Kate, and Prince Harry and Duchess Meghan unexpectedly appeared together to greet a crowd outside Windsor Castle.
Earlier this month, the brothers attended an awards ceremony in London honoring their late mother, Princess Diana, but they were oceans apart.
Prince William spoke in person at the Diana Legacy Awards, while Prince Harry addressed the event via video link from California after his brother left.