AceShowbiz -

Blac Chyna being part of Khloe Kardashian's 32nd birthday party came as a surprise. The former "Kocktails with Khloe" host was one of Rob Kardashian's sisters who disapproved of his relationship with the pregnant model, so it's only natural for people to wonder how Blac could make the guest list.

Now, a new report has emerged claiming that Khloe originally didn't plan to invite Blac, but then changed her mind after her brother Rob gave her an ultimatum. "Rob and Khloe are as tight as a brother and sister could possibly be, but he wasn't going to her birthday party unless Blac was coming," a source told HollywoodLife.com.

"Khloe initially didn't want Blac to attend because she still holds a grudge against her for attacking Kylie [Jenner] and believes her intentions with Rob are not on the up and up," the source explained. "But Rob hit Khloe with that reverse psychology, telling her that, if she's all about family like she always proclaims, then she needs to respect Blac as part of the family, because in due time, she will be. Khloe budged and invited Blac."

Khloe, who lived with Rob and was super close to him before he began dating Blac, was livid when she found out her brother got engaged to the mother of one and would be a dad to another baby she's pregnant with. However, the youngest Kardashian sister looked happy when posing for a picture with the pair at her birthday party on Monday, June 27.

The other members of the Kardashian-Jenner clan also seemed to have lots of fun hanging out with Blac. She snapped some selfies with Kim Kardashian and Kris Jenner, and sang karaoke with Kylie Jenner at the star-studded party at Dave & Buster's in Hollywood, California.