Kim Kardashian, Kendall and Kylie Jenner Wear Matching "Mason Hearts Penelope" Necklaces

Rapper Kanye West and TV personality girlfriend Kim Kardashian made their exit after shopping at the Westfield Topanga Plaza in Woodland Hills, California on July 9, 2012. Credit: FAMEFLYNET PICTURES

The Kardashian krew is keeping their newest addition -- and her older brother -- close to their hearts. Literally.

Kim Kardashian and her half-sisters Kendall and Kylie Jenner have been spotted rocking necklaces that read "Mason <3s Penelope" since older sis Kourtney Kardashian, 33, gave birth to her daughter with Scott Disick on Sunday.

Penelope Scotland Disick is the couple's second child; they are also parents to Mason, 2.

On Monday, Khloe Kardashian took to her blog to gush over the newest Kardashian family member -- and how Mason is liking brotherhood so far.

"The entire family is literally exploding with joy and have not stopped fawning over our new little girl," the reality star wrote. "And Mason is already loving being a big brother!"

Kim, 31, also blogged about her adorable new niece.

"[Penelope] is absolutely perfect!" she shared. "I am so excited to be an aunt again . . . I know Mason is going to be the best big brother in the whole world."