Kourtney Kardashian's son Mason has already exhibited signs of becoming "a super protective big brother."

The reality TV star and her partner Scott Disick welcomed their second child daughter Penelope Cruz on Sunday July 8.

Mason loves his little sister immensely already.

"He's really excited about having a new playmate and has actually showed an incredibly mature side," said a source.

Kourtney and Scott are intent on making family decisions with Mason.

The toddler helped decorate his sister's nursery.

"Kourtney let Mason pick out some furniture," the source revealed.

"Kourtney and Scott have been pretty sensible about it. They included Mason in decisions about the nursery and everything else that's been going on. He's quite fascinated by all the new girly toys. The only problem was that Kourtney wanted to baby's nursery to be white and Mason's favourite colour is blue!"