Kim admitted she quizzed Tristan - who was later revealed to have been unfaithful again and fathered a child with Maralee Nichols last year - about proposing to her sister and was stunned when the 31-year-old sportsman revealed he already had.
The Skims founder said: "The fact that he proposed and you never told us. And I asked him months later … the night after I thought it happened and he's like, 'Oh, I did it months ago, she didn't tell you guys?' "
In a confessional, the aspiring lawyer went into more detail about the chat with Tristan after all the "drama" surrounding his infidelity.
Kim said: "I'll never forget Tristan calling me and he was gonna propose to her on Valentine's Day, which was a year since they had drama so this was last Valentine's Day.
"And then I called him the day after and I was like, 'Hey, I never heard anything from Khloe. Oh my god, did you propose?' and he was like, 'Oh yeah, I did in December.' Oh my god, she didn't tell us!"