We thought big brothers were supposed to protect their little sisters -- not post their private information for the world to see!

Rob Kardashian once again flew off the handle on Monday -- and took his rage out on his sister via Twitter, posting his sister Kylie Jenner’s phone number on the social media site after becoming irate over his family's baby shower.

He didn't do it just once -- he posted the digits TWICE.

The soon-to-be dad wrote, “Kylie’s #….I ain’t hacked either this is rob dog lol."

Apparently the spiteful move was a result of the hot-headed reality star's ire over his baby mama Blac Chyna not being invited to the Kardashian's baby shower for his unborn daughter.

Rob wrote, “Didn’t invite the Mother of my child to a baby shower you were trying to throw me !!? You all must have lost your damn mind.”

Wow -- taking out your frustration on your teenage sister? That's messed up.

Since the hoopla happened Kylie's disabled her old phone number so the number no longer works.

Maybe this is just all a way to promote Rob & Chyna, which airs on E! on Sundays at 9 p.m. ET. Whatever's going on -- Rob's got major problems!