The 28-year-old reality TV star - who has been battling weight gain and depression for months
- hadn't posted on the social media website since 29 May but came back to his account on Tuesday.
He returned to Twitter to give his 34-year-old sister - who is pregnant with her second child with husband Kanye West - some advice for the nausea she's been experiencing.
When Kim tweeted that she "thought this nausea thing was supposed to be gone by now", Rob replied: "I sent u that voice note. It should help."
Wait wait wait I thought this nausea thing was supposed to be gone by now!!!! UGHHHHHH
The Keeping Up With The Kardashians star then replied: "Should I post it? I think people would love to hear the voice notes u send me! (sic)".
While it's unclear what the voice note included, Rob later referenced Khloe Kardashian's ex French Montana's lyrics from Hard Work.
Rob's last tweet to his older sister - who is due to give birth to a son later this year - compared her to that "bitch from Gone Girl", so the siblings have obviously made up.
A source close to the Kardashian family previously said: "He has a funny sense of humor and posts weird things and then deletes them. It's just him."