The future British king - who will wed fiancee Kate Middleton on April 29 at Westminster Abbey in London - wants his younger sibling to be at his side on his big day as well as giving him the responsibility of organising his stag party.
While William has chosen his brother, Kate has opted for her sister Pippa Middleton to be her Maid of Honour.
It seems William and Kate are keeping it in the family as the prince's seven-year-old cousin Lady Louise Windsor - the daughter of Prince Edward and his wife Sophie, Countess of Wessex - has been asked to be a bridesmaid.
A Buckingham Palace source said: "Louise is thrilled to be chosen. She is absolutely cock-a-hoop and can't stop chattering about it. She'll look absolutely radiant on the day. Her mother is so proud."
Kate's other bridesmaids include Viscount Linley's daughter Margarita, eight, Eliza Lopes, three, and William's three-year-old goddaughter Grace Van Cutsem.
William Lowther-Pinkerton, 10, and William's eight-year-old godson Tom Pettifer will act as pageboys.
A source said: "It might be a global event but William and Kate still see this as a proper Family Affair. They don't want to offend anyone and don't want to leave anyone out."