Rob Kardashian Posts Rare Instagram for Kylie Jenner's Lip Kit, but Not Kim Kardashian's New Baby

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Rob Kardashian is apparently letting his social media do the talking.

The 28-year-old former Keeping Up With the Kardashians star posted a rare Instagram shot on Saturday, promoting his younger sister Kylie Jenner's sold-out lip kit. Curiously, he made no mention of a much more significant event that happened in the Kardashian family, although the news broke the same day -- Kim Kardashian giving birth to her second child with husband Kanye West.

"I guess someone is disappointed they didn't get Kylie's lipkit!!!!" Rob wrote next to a picture of his dog on a laptop looking at 18-year-old Kylie's lip kit online. "Don't worry buddy more coming soon!"

WATCH: Kim Kardashian Gives Birth to Baby Boy With Kanye West!

It's no secret that Rob's relationship with his famous family has been tense over the past few years. Last week, his mother, Kris Jenner, revealed that her only son didn't spend Thanksgiving with the Kardashian clan.

"He lives with Khloe," Kris noted on Maria Menounos' Sirius XM radio show, since her 31-year-old daughter hosted the holiday dinner this year. "So he was around, and then went over to a buddy's house."

"He's doing alright, he's doing his best," Kris added about Rob. "He's working on himself. He's gonna be OK. ... He's not as into being on camera as the rest of us."

Rob has struggled with depression and weight issues for the past two years. In October, Khloe Kardashian -- who's widely thought of as being the closest sister to Rob -- admitted that even her patience for her brother/roommate has been tested.

"I've been working on my patience. I've been really trying," Khloe told People. "But I have my moments and I do snap. I've offered to get a chef, I'm like, 'What's your excuse?' But it's not just that. I can't win life for Rob. I have to wait until [he's] ready to do it for himself."

"It's sad, because he's only wasting his life more and more," she added. "But he has to figure it out for himself. And he has so many people who want to help him."

Though perhaps Rob's social media silence on Kim's new baby isn't all that surprising given their history. Rob has made his bad blood with Kim known before -- he compared her to "the b**ch in Gone Girl" in an Instagram post in March, which he later deleted -- and he also notably left Kim and Kanye's Florence wedding last May. Kim has previously insisted on Keeping Up With the Kardashians that their conflict is due to her being the only one in the family to show him some tough love.

"He obviously has some kind of depression," Kim said in an April episode of the E! show about Rob, who, according to Kris, has gained 100 pounds over the last year. "He is the king of excuses ... I'm the only one that's really tough with him so he probably avoids me the most."

Still, Rob did take some time out to publicly wish Kim a happy birthday in October.

"Happy birthday Kimburrrrr, thanks for always feeding me. Cheese fries all day baby," he wrote alongside a screenshot of the two eating from an old Keeping Up With the Kardashians episode.

PHOTOS: Scott Disick and Tyga Spend Quality Time Together While Kardashian Family Visits Kim and Her Baby

Check out how the rest of the Kardashian family celebrated the birth of Kim and Kanye's son in the video below.

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