Kim Leadbeater entered Parliament with a beaming smile and a thumbs up as she took her seat in the House of Commons.
The new Labour MP for Batley and Spen was also hailed for her "bravery" following a bitter and divisive election campaign.
She follows in the footsteps of her sister, the murdered MP Jo Cox - winning the election by 323 votes.
Ms Leadbeater was accompanied by party colleagues Shabana Mahmood and DanJarvis as she arrived in the chamber.
She opted to affirm her allegiance to the Queen during the brief swearing-in ceremony and then gave a thumbs-up to MPs on both sides of the chamber.
Ms Leadbeater then walked straight over to Sir Lindsay Hoyle without signing her name on the test roll - a parchment book which was originally intended to prove a new member's loyalty to the Crown.
Speaker Sir Lindsay told her: "Don't worry, we'll have a chat in a minute."The pair then exchanged a few words before Ms Leadbeater exited the chamber.
Defence Secretary Ben Wallace told the Commons: "Could I just congratulate the honourable member for Batley and Spen for taking her place and also say my personal admiration for both her bravery and sense of duty in putting herself forward to stand for that seat after the tragic loss of her sister."
Outside Parliament the new MP looked jubilant as she was welcomed by party leader Sir Keir Starmer and her Labour colleagues.
Looking and sounding slightly emotional Ms Leadbeater said she was incredibly 'honoured and proud' to be MP for Batley and Spen, adding there was a 'lot of work' to do both locally and nationally