"Khloe Kardashian, I’m so happy for you!" she wrote. "Your baby girl is so beautiful! You are so strong, you made that look so easy!"
Kim recently welcomed a daughter of her own, Chicago, in January, and is thrilled that her and Khloe's daughters are so close in age. "I can’t wait for our baby girls to grow up as best friends the way we are!" she continued.
The mother of three also revealed that she had a part in helping Khloe, 33, while she was in labor. "I’m so happy I was able to return the favor of holding your leg while you pushed LOL," she mused.
💕 @khloekardashian I’m so happy for you! Your baby girl is so beautiful! You are so strong, you made that look so easy! I can’t wait for our baby girls to grow up as best friends the way we are! I’m so happy I was able to return the favor of holding your leg while you pushed LOL
Kim wasn't the first of her family members to congratulate Khloe.
Kylie Jenner -- who became a new mom in February when she gave birth to daughter Stormi -- posted a sweet message on Snapchat for her big sis. "Kongrats, Khloe! It’s a girl!” she captioned a scenic photo taken from a car.
Kim, along with her mom, Kris Jenner, and sister, Kourtney Kardashian, flew to Cleveland, Ohio, to be with Khloe when she gave birth.
Just following her delivery, Kris took to Instagram to share a funny video of daughter Kendall Jenner exclaiming, “Baby, baby, baby!” She captioned the post: “BLESSED!!!”
The happy news of Khloe's daughter comes just a few days after video surfaced of her boyfriend and the father of her child, Tristan Thompson, allegedly kissing another woman at a New York City nightclub.
"It wasn’t exactly how we expected it to happen," a source close to the Kardashians told ET, adding that the family is united at this time and thrilled for Khloe and her baby. "But for now, all anyone cares about and is talking about is the baby. Everyone is thrilled! Despite everything else going on, this is an incredibly happy day. The drama isn’t going to overshadow that."
Here's more details about Khloe's birth and her relationship with Tristan: