What were the Kardashian and Jenner sisters doing when Rob announced engagement to Blac Chyna?

Rob Kardashian, 29, seems smitten by make-up artist Blac Chyna a.k.a Angela Renee White. The couple announced their engagement in a seemingly casual photo on Instagram Tuesday, April 5.

In the picture, Chyna, 27, is seen caressing her boyfriend's chin while sporting a huge diamond rock. News of their engagement came as a sweet surprise to many, but Rob's family, the Kardashians and the Jenners have been largely silent.

Rob's sisters Kim, Khloé and Kourtney Kardashian and Kylie and Kendall Jenner's social media accounts do not include an acknowledgement of their brother's engagement. Their silence is stark, since they have always used their Twitter or Instagram accounts to post inspiring and loving messages to Rob, who battled weight issues earlier.

The famous reality stars, who are presently on a secret family vacation with momager Kris, have been uploading several pictures of their trip.

In an Instagram post Tuesday, Khloé uploaded a picture of her and niece Penelope Disick. She captioned the picture: "I made her my mini me!!"

Meanwhile, Rob's mother Kris, posted a picture of boyfriend Corey Gamble with family member Todd Kraines on her Instagram account.

Fans of Kanye West's wife, Kim, asked her to congratulate Rob on his engagement. The "Keeping up with the Kardashians" star's tweet about festival accessories was met with angry responses.

So far, it is only Kylie's boyfriend Tyga who made a statement about his ex, Chyna's engagement. The rapper, who shares a son with Chyna tweeted, "Everybody deserves 2 be happy. What some1 does for their happiness is not my concern, as long as it's not interfering wit my happiness."

News of Rob and the make-up artist's engagement did shock the Kardashian sisters, but wasn't unexpected, a source told People Magazine.

According to the source, "There has been a fear that Blac Chyna is manipulating the situation with Rob. Blac Chyna has been great as kind of a rebound. As someone that got Rob out of a dark place, that helped him get motivated and healthy again. Let's just be honest, no one is really hoping for a marriage to happen."

Rob, who proposed to Chyna Monday, April 5 at the Ace of Diamonds strip club in Los Angeles, has been dating her since January 2016. Earlier, Khloe addressed her brother's relationship and her alleged fight with Chyna on her TV show, "Kocktails With Khloé."

She said, "My brother is dating Blac Chyna. My sister, Kylie [Jenner], is dating Tyga, who is Chyna's baby daddy," Khloe explained. "I need a piece of paper to show you guys because it's confusing! It's like a soap opera, so it sounds very dramatic. Which it is," according to E! Online.