Sienna is the youngest daughter of Olympic gold medallist Peter Charles and the sister of Harry Charles.
She represented Great Britain on four occasions as a youth athlete, winning team silver at the FEI Jumping Junior European Championship in 2018 and team bronze at the FEI Jumping Young Rider European Championship in 2021.
In 2021, she was crowned U25 National Champion and was British Showjumping's Leading Young Rider in 2023.
Sienna made her senior team debut at the FEI Jumping Nations Cup at Falsterbo, Sweden, where she produced a clear round to help Great Britain finish second.