She's gone from the most boring sister to the biggest headline grabber!

Kourtney Kardashian took daughter Penelope and son Reign to a playdate in Beverly Hills this afternoon, and the reality star looked like she was feelin' herself in black leather pants, a black turtleneck, and shades. Her body is looking better than ever, but we think the pep in her step has more to do with the fact she's hooking up with Justin Bieber! Who wouldn't get a confidence boost from that?!

Although Kourt's not the only chick he's been getting busy with, it seems that JB is pretty into the mother-of-three. "He couldn't believe that she would go for him. He brags about how hot she is. He is used to being swooned by young girls that have nothing else going on. Kourt is a hot mom with a career. He is happy while it lasts. Even if she is using him to get back at Scott, he couldn't care less," a source dished to People.

Speaking of the Lord, he's allegedly pissed even though he knows can't do much about it, given his cheating scandal this past summer.

"Of course it stings a bit that Kourt is hooking up with Bieber. But Scott also knows his place and that he doesn't have the right to have a fit about it. He is convinced Kourt is doing it to hurt him. And she succeeded," another source told the mag.

Possibly the most hurtful part of all of this for LD? The fact that the "Sorry" crooner is dropping by the family home to hang out with his children! "He actually goes over a lot to see her kids, and they love him," a source told Us Weekly.

It's hard to tell who is more upset about this romance -- Scott or Kim? After all, poor Kimmy K and her newborn son Saint are being totally overshadowed by her sister's bone sessions with The Biebs!