Sofia Richie was ‘surprised by how much fun’ she had at Kourtney Kardashian’s birthday party, but she had a ‘secret weapon.’ No, not Scott Disick — Paris Hilton bridges ‘the gap’ between the Kardashians and Sofia.

Sofia Richie, 20, didn’t expect to have so much fun at Kourtney Kardashians’ 40th birthday party on April 18, and she can thank her honorary older sister, Paris Hilton, 38, for that. “Sofia had the best time at Kourtney’s party, everyone welcomed her with open arms, she is totally one of them now. And not only was Kourtney and the whole family super warm to her, she also had a secret weapon there — her lifelong friend and supporter Paris Hilton,” a Sofia source EXCLUSIVELY tells HollywoodLife. Paris starred on the jewel of MTV, The Simple Life, with Sofia’s older sister Nicole Richie, 37, from 2003 and 2007, and that history hasn’t been forgotten.

“Paris is family to Sofia, like a big sister who has seen her grow up and been there for her every step of the way, so her being at the party made it even better,” our source explains. And it was the hotel socialite who acted as the mediator between Sofia and the Kardashian sisters, who were hesitant at first to accept the much younger girlfriend of Scott Disick, 35 (you know, Kourtney’s ex and the father to her three kids). “Paris has really bridged the gap between the Kardashians and Sofia,” a second source, who’s a friend of the Hilton and Kardashian families, EXCLUSIVELY tells HollywoodLife. “At first they were very closed off to her, but now, with the Hilton family’s help, they see just how mature and wonderful Sofia is for Scott.” Paris has reconnected with the Kardashians recently, as she attended the Kardashian Christmas Eve party for 2018, while Kim attended Paris’ 38th birthday party on March 16.

Paris wasn’t the only familiar face at the birthday bash on Thursday night! “Although Sofia has come a really long way in building a relationship with Kourtney, she still felt somewhat nervous about attending Kourtney’s birthday party. Sofia realized what a big step forward the invite was, not only to her, but to Scott, so it was really important to make a good impression and fit in with Kourtney’s friends and family. Once Sofia arrived and saw that some of her own friends were already there, it definitely helped relieve some of the anxiety,” another Sofia source EXCLUSIVELY tells HollywoodLife.

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“Sofia was able to just let loose and have a good time. She was surprised by how much fun she actually had and was so grateful Kourtney welcomed her with open arms,” our third source continues. Sofia showed up in an outfit that served as the perfect party ice breaker, as she stunned in a high-slit gown, fastened by a gold horseshoe ring, from Jill Jacobs.