Kourtney Kardashian better learn to share, because sister Kim Kardashian can't get enough of her new niece!

PHOTOS: Kourtney's maternity style

Kourtney and boyfriend Scott Disick welcomed their baby girl, Penelope Scotland Disick, on Sunday. Since then, the whole Kardashian family has been celebrating the new addition on Twitter.

On Tuesday, Kim, 31, shared her excitement once more, tweeting, "I'm so in love with my niece!"

PHOTOS: Inside Kourtney's baby shower

Kim is already an aunt to Kourtney, 33, and Disick's 2-year-old son Mason. "She is absolutely perfect!" Kim wrote on her blog Monday about her niece, Penelope. "I am so excited to be an aunt again and can't wait to spoil little Penelope. I know Mason is going to be the best big brother in the whole world. Congratulations Kourtney and Scott."

VIDEO: Kourtney breaks the baby news to the family

The rest of the Kardashian family has been teasing tweets this week that led fans to believe they'll finally get to see a photo of baby Penelope.

Khloe Kardashian tweeted Tuesday, "Look at that baby face!" But the photo turned out to be an old baby photo of her sister Kendall Jenner. And Kendall pulled the same prank on Monday by tweeting, "Baby!" That image turned out to be photo of a puppy.

This article originally appeared on Usmagazine.com: Kim Kardashian: "I'm So in Love With My Niece!"