Now, we finally get to see proof of their shock and disgust.
"He proposed in front of her whole family? That's so f----- up," Khloe says when she hears the news. We hear one of the other sisters yell, "Nooooo!"
The clip also shows Rob and Kim in a very tense telephone conversation, in which he tells her that he doesn't want there to be any fighting between his sisters and his lady love, who has been a family frenemy in the past.
"You make sure your girlfriend doesn't publicly bash your sisters!," Kim tells him about her one-time pal.
Khloe also seems baffled that her once-reclusive brother is suddenly comfortable with being so public.
"The fact that he now wants to be so public blows my mind," she says.
The video ends with Rob having a combative sit-down with his three older sisters.
"Listen to the hostility in your voice, that's exactly why I don't say anything," he tells them. "This is the first time I ever proposed to someone in my life, don't come at me with some bulls---!"
It's not known when exactly the footage was filmed, but it seems that the family has come to terms with Rob's engagement and gotten past the hard feelings.
On Thursday, Khloe shared a photo of her and her brother holding hands on Snapchat.
Another shot of the new besties was captioned, "Hanging out with my lil sis."
Rob, too, couldn't help but gush over the two women seemingly letting bygones be bygones.
"So happy my girls finally got to talk and make peace! #MyFamily #AllLove," he captioned an Instagram snap of Kylie and Chyna.