The fallout from Rob Kardashian and Blac Chyna‘s sudden engagement continues as the Kardashian-Jenner clan grapples with the devastating news.

“It’s the worst betrayal ever, especially since Blac has badmouthed the family so many times,” an insider tells Radar.

“The family thinks Rob’s doing this to piss everyone off because he’s so angry,” added the source.

In the wake of the news, behind closed doors the reality family is beginning to fight back against their rogue relative.

“There is now an ultimatum in place to deal with this mess,” revealed the insider. “He’s going to be dropped from the family if he marries her.”

As for momager Kris Jenner, 60, and the rest of her daughters, “They don’t really know if Rob’s feelings for Blac are even sincere at this point,” said the insider, “but they are questioning his sanity and don’t think he’s thinking straight.”