The 'Keeping Up with the Kardashians' star is close friends with the 23-year-old model, and has said that although she was surprised when Justin proposed in July 2018, she wanted them to tie the knot ''so bad''.
During an Instagram Live with the married couple - who wed in private in September 2018, before hosting a larger ceremony a year later - Kendall was asked if she saw their relationship coming, and she said: ''Honestly, no!''
But then added: ''I think I hoped, obviously Hails is my lady, my best friend, and I wanted it so bad for both of you. It just makes me really happy.''
Kendall, 24, also spoke about gushing over Justin and Hailey's romance at their wedding, which she admitted she was ''pretty drunk'' at.
Justin told her: ''Through the ups and downs you've always been there, I know you have for Hailey.''
And when he suggested Kendall would take Hailey's side if the couple were ever to split, the model said: ''No, I love you both, but Hails is my girl.''
During the video, the 'Yummy' hitmaker, 26, also asked Kendall and Hailey how long they'd known each other, and Hailey traced back their friendship to the premiere of 'The Hunger Games' in 2012.
And Kendall added: ''But you were more friends with Kylie [Jenner, Kendall's younger sister]. Oh my God, I was so young. I was friends with Ireland [Baldwin, Hailey's cousin], then you and Kylie bonded, then I stole her! I stole Hails! She was my first real friend in New York, Hails is the reason I love New York.''
To which Hailey squealed: ''I love that! I've never heard you say that!''
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