From big brother to best friend and now best man, Prince William is slated to take on the role as Prince Harry's best man in his 2018 wedding to Meghan Markle.

Axar.az reports citing E! News that the betrothed royal has already asked William to be his best man when Harry and Meghan tie the knot at St. George's Chapel at Windsor Castle this May.

The source adds that Kate Middleton will not have a role in the nuptials, as she will have her hands full with three children (she's due to give birth in April) and plans to stay in the pews for the ceremony.

Speaking of children, the insider informs E! News that Harry's niece and nephew, Prince George and Princess Charlotte, will have roles on the day of the wedding, as will Isabel Veronica Mulroney, the daughter of Meghan's best friend Jessica Mulroney and husband Ben Mulroney.

Traditionally, there isn't a "best man" at a royal wedding. Instead grooms have "supporters." Prince Charles asked brothers Prince Andrew and Prince Edward to be his "supporters" when he married Princess Diana in 1981.

However, Prince William broke with tradition and had Harry be his best man when he married Middleton in 2011. Kate's sister Pippa Middleton served as maid of honor.

Clarence House announced last Monday that Prince Harry and the Suits star were engaged.

The 33-year-old royal asked the 36-year-old American actress to marry him while cooking a chicken dinner at home in London. The proposal came a year and a half after they started secretly dating.

At the time of the proposal, E! News reported that Prince Harry has informed Queen Elizabeth II and other close members of the royal family, and he asked Meghan's parents' permission before he proposed.

The wedding will occur at the 14th century chapel that has been home to a long line of royal weddings beginning in 1863 and continuing into 2005, where Prince Harry's father, Prince Charles, and his stepmother, Camilla, Duchess of Cornwall, received a blessing on their wedding day.