Kim Kardashian couldn't be more thrilled about her big sister Kourtney's baby.
"I'm an auntie!!!!" she posted on Twitter Monday shortly after Kourtney and Scott Disick welcomed their baby boy, who weighed in at 7 lbs., 6 ozs.
"My little Mason Dash Disick is here!!! We are so happy!" she added.
Last week, Kim went Christmas shopping for Mason at NYC's FAO Schwarz.
"I think we are going to do a little bit of baby shopping because we have no idea what to get Kourt's baby that will be here by Christmas," she told "Extra." "So we're hoping to find some of Kourtney's baby's first toys."