It's For Real! How Kris Broke The Rob Engagement News To His 'Livid' Sisters

It's not a joke! Rob Kardashian proposed last night to his girlfriend of only two months, Blac Chyna, with a gigantic diamond sparkler – and she said yes!

But before the big ask, RadarOnline.com has exclusively learned, momager Kris Jenner plotted with her son to break the news to his sisters in a way that would ensure damage control: by telling them while on vacation!

As previously reported, the 60-year-old matriarch jetted out of Los Angeles on Sunday with all of her ladies in tow. Yes, that means ALL of them – Kim, 35, Kourtney, 37, and Khloe Kardashian, 31, as well as Kendall, 20, and Kylie Jenner, 18.

"The real reason that Kris took all the girls on this 'surprise' getaway is because she needed to break the news to them that Rob was going to propose to Blac Chyna," said the source, adding, "Kris is actually very happy for her son but she knew that his sisters were going to lose their sh*t when they found out."

And apparently, they did!

"Kim, Khloe and Kylie are without words," the insider told RadarOnline.com. "Honestly the only ones that are happy for him are Kourtney and Kendall because they have always stood behind their brother."

As RadarOnline.com previously reported, Blac Chyna, 27, is the baby mama to Kylie's 26-year-old bad-boy rapper boyfriend Tyga's son, King Cairo, 4.

In the most tangled family tree ever, the union of Rob and Blac Chyna will make Tyga his son's uncle!

"Kris is demanding that they ALL be supportive of this because they have to be," the source added. "Whether they like it or not, Blac Chyna is going to be a Kardashian."