The 33-year-old television personality dedicated a sweet post to Kourtney Wednesday on Instagram.
Khloe shared a slideshow of different moments with Kourtney, including a pillow fight, workout session and a childhood photo. She said in the caption that she "can't imagine" life without her eldest sibling.
"Happy birthday @kourtneykardash!!!!" the star wrote. "In any lifetime would have found a way to have you in my life forever! I can't imagine life without you! Have the best birthday!! You deserve everything and more! Forever and always I love you!!!"
The tribute marked Khloe's first post since announcing her daughter's name, True Thompson. Khloe welcomed the infant with Tristan Thompson last week following rumors Thompson cheated during her pregnancy.
"She is absolutely not even thinking about her relationship with Tristan," a source told to Us Weekly on Wednesday. "The primary concern Khloe has is that Tristan and True are bonding."
"Nothing has been decided," another insider added. "She has not broken things off with Tristan ... This is all still very new and no decisions have been made."
Kourtney's mom, Kris Jenner, and siblings Kim Kardashian and Rob Kardashian also wished her a happy birthday online. E! News reported Kourtney, who shares three children with ex-partner Scott Disick, spent her birthday with her kids.