Lucy Lanigan-O’Keeffe, the assistant head teacher at George's school in London, is marrying their dad's life-long friend, Thomas van Straubenzee. Charlotte will also start attending the school, Thomas' Battersea School, later this year.
Van Straubenzee, 36, proposed to Lanigan-O'Keefe, 31, last week, according to The Sunday Times. It is not clear who first introduced the pair.
Prince William will be an usher on the big day, while George and Charlotte are expected to be a pageboy and bridesmaid, The Sunday Times reported.
Prince William has known van Straubenzee since he was young; they attended the Ludgrove School in Berkshire together.
Van Straubenzee and his younger brother, Charlie, both served as ushers at Prince Harry and Meghan Markle's wedding last year. He is also Charlotte's godfather.
It's not the first time Prince William and Kate Middleton's children have taken part in a wedding. They were page boy and bridesmaid for Prince Harry and Meghan Markle, as well as for Princess Eugenie when she married Jack Brooksbank last year. They also took part in the wedding of their mother's close friend, Sophie Carter, in 2018, and the wedding of their aunt Pippa and James Matthews in 2017.