This isn't the first time Prince George's little sister has worn the brand - Pepa & Co are same company that made the together page boy and flower girl outfits for aunty Pippa Middleton's wedding to James Mathews in 2017.
Queen Elizabeth II and Prince Philip were not in attendance.
While many eyes were on the 11-week-old royal baby who has not been seen in public since his birth on April 23, fans of the famous family were also curious to see what Kate would be wearing to such a special occasion.
Louis's aunt and uncle the Duke and Duchess of Sussex were among the guests in the chapel at St James's Palace in London.
While walking hand-in-hand with her father, the Duke of Cambridge, Charlotte was heard telling photographers: "You're not coming!" as the family made their way to Louis' christening tea.
At Prince Louis' christening this afternoon at St James's Palace, the three-year-old was spotted hand-in-hand with her parents as she made her way into the chapel for the service.
An old friend of Prince William and Prince Harry, the 36-year-old will be attending the christening in London among friends and family of the Duke and Duchess of Cambrige.
They were joined by Prince Charles and the Duchess of Cornwall.
The robe, which was handmade by Angela Kelly, the Queen's personal dressmaker, is a replica of the original christening gown designed for Queen Victoria's eldest daughter. She is friends with Kate from their school days at Marlborough College.
The Palace confirmed that the hymns were, "O Jesus, I Have Promised" and "Lord of All Hopefulness", with the lessons being from St. Mark 10: 13-16, read by Miss Lucy Middleton, and Ephesians 3: 14-19, read by Mr Guy Pelly.