The family drama is at an all-time high on Sunday's new episode of Keeping Up With the Kardashians.
Rob, 29, says the proposal happened "spontaneously," and also gets defensive about not confirming the news to his family immediately.
"If you guys really were concerned and wanted to know, you guys would have said, 'Is this true? Congratulations, you just got engaged,'" Rob says, raising his voice. "You think I'm just going to post some fake s**t like I'm a media w**re? Never in my life."
"It's a two-way street," he adds. "You guys could easily have picked up the phone and called me. No, I'm not going to talk to you guys when you're in a room with Tyga, and on a ski trip with other family members."
"You guys act like I'm just gonna call you guys and say, 'Oh, this and that,' when you hate this girl," Rob points out.
"But we love you," Kim fires back.
Though eventually, Kim admits that Chyna has been a good influence on her formerly reclusive younger brother.
"We see that you're happy, and that's all that matters to us," Kim concedes. "We obviously have tried to get you out of the house for years, you know, and that hasn't worked. And, so, if someone else could do that, we are really thankful for that."
Kourtney is similarly diplomatic.
"Of course we would be happy for you," Kourtney tells Rob. "But we're allowed to be hurt that you didn't tell us, but you're allowed to do it in your own way."
Rob later acknowledges his sisters' hurt, and apologizes.
"I wasn't even thinking that at the time. It happened so fast," he says about not keeping them in the loop about his engagement. "I get I upset you guys. I'm your only brother, you would like to know when something important like this happens. I apologize because I wasn't trying to be rude."
These days, it appears to be all good between Chyna and the Kardashian family. Rob and Chyna attended Khloe's 32nd birthday party at Dave & Buster's
in Los Angeles on Monday night, where the 28-year-old pregnant model shared happy snaps posing with the birthday girl, as well as Kim, and Rob's mother, Kris Jenner.