The two girls joined their fellow siblings, Kourtney, Rob and Kim - who broke her Twitter silence since having baby North West - to send messages of love and well wishes to Khloe, who turned 29.
Confusingly, Kylie referred to Khloe as her "mommy" on Twitter, which for a second had us thinking that Kardashian matriarch and 'momager', Kris Jenner, was also celebrating her birthday.
Kylie tweeted: "Happy birthday to my mommy!"
However, the 15-year-old soon cleared things up on her joint blog with older sister Kendall by explaining that Khloe was their "second mother" and had supported them greatly throughout their lives.
Kylie and 17-year-old Kendall then posted a sweet message along with a collage of old snaps of the three sisters together over the years.
They wrote: "Happy Birthday to our amazing sister and second mother Khloe Kardashian!!! We love you with all our hearts and can't thank you enough for all your love and support throughout the years! You have truly been an inspiration to us our entire lives. Can't wait to celebrate your birthday Khlo! Xoxo Kendall and Kylie".
As well as the sentimental blog post from her younger siblings, Khloe was inundated with birthday messages and tweets from the rest of her family and her fans, which she admitted had made her feel pretty emotional and even made her to cry!
She tweeted: "I guess this is what happens when you get older. I have been crying reading some of the sweetest tweets for my birthday. So sweet! Blessed!
"PS... I'm not a crier so this is a big, weird, silly thing for me :)"
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