Kate Middletonis continuing tocement her role as future queenas she helps her grandmother-in-lawQueen Elizabethentertain leaders from around the world, includingPresident Donald Trump, on Tuesday evening.

Kate, 37, is without husbandPrince William, who is currently on a royal tour in Kuwait and Oman, but she joins other senior members of the royal family for the Buckingham Palace reception, which marks 70 years of NATO.

The royal mom, who opted for a bright green ensemble for the occasion, was spotted on her way to the palace reception after leaving her Kensington Palace home.

Multiple sources tell PEOPLE that they have observed agrowing confidence in Princess Kate, 37. While raising her three children withPrince William, she has found passion and purpose in helping mothers and children everywhere. “She is finding her voice,” says a well-placed palace source.

Prince Charlesand his wifeCamilla, Duchess of Cornwallwill also be alongside the Queen, 93, to formally receive the NATO leaders and their partners before a formal photograph of the royals will be taken with the various heads of state and government.

YUI MOK/POOL/AFP via Getty

Buckingham Palace reception

Kate Middleton.

Kate Middleton

Kate Middleton arrives at palace reception.

Kate Middleton

Queen Elizabeth.

Buckingham Palace reception

Notably absent from the reception isPrince Andrew, 59, who has stepped back from public life in the wake of his bombshell interview abouthis friendshipwith convicted sex offender Jeffrey Epstein.

Chancellor of Germany Angela Merkel talks to Queen Elizabeth at a reception for NATO leaders at Buckingham Palace.

Buckingham Palace NATO reception

Queen Elizabeth and NATO leaders.YUI MOK/POOL/AFP via Getty

NATO heads

Two royal family members who are also not in attendance areMeghan MarkleandPrince Harry.The Duke and Duchess of Sussex are missing the event as it falls during their “family time” off from royal duties. The new parents previously revealed that they would betaking about six weeks offtowards the end of the year amid their concerns that theintense scrutinythey’ve faced has been tough on the couple.

Kate Middleton.YUI MOK/POOL/AFP via Getty

Kate Middleton

Meghan, 38, and Harry, 35, are also planning toskip the annual holiday festivitiesat Sandringham — including their public walk to church on Christmas morning.

Princess Anne.Yui Mok - WPA Pool/Getty

Buckingham Palace NATO reception

From left: First Lady Melania Trump and President Donald Trump at Buckingham Palace on Tuesday.Dan Kitwood/Getty

Melania Trump, Donald Trump

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Tuesday’s reception is one of the central showpiece events of the two-day meeting that takes place on Tuesday and Wednesday and is significant because it helps celebrate the 70th birthday of the alliance. NATO was established in 1949 with 12 founding members, and is now a collective defense agreement spanning of 29 countries. The United Kingdom is one of NATO’s 12 founding members and London was home to its first headquarters.

source: people.com