The 31-year-old reality star let her 28-year-old sibling know she's thinking of him by sharing a throwback photo of herself with Rob on Thursday.
Rob is wrapped up in girlfriend Blac Chyna, who he's been linked to since January. The 28-year-old has been more active on social media since he started dating the 27-year-old model, and was all smiles in a recent Snapchat.
"Rob has fallen hard for Blac Chyna," a source told People magazine. "She makes him super happy and this is the first time that he's felt so optimistic about life in years."
"She helps keep Rob in check with his health," the insider added. "He feels there's strong potential for this relationship to go a long way but for now they are very content and complement each other really well."
Rob was diagnosed with diabetes in December after gaining 100 pounds over the past couple years. He had largely retreated from the spotlight by the time of his hospitalization, and Khloe remains one of his staunchest supporters.
"If only you could see yourself through my eyes! I'll forever be obsessed with you! My one and only! #MyAce," she tweeted prior to Rob's diagnosis.
Khloe is also caring for estranged husband Lamar Odom, who made his first public appearance since his own hospitalization at Kanye West's fashion show last week. The reality star said she and Lamar "take every day as it comes" in January.