On Sunday's Keeping Up With the Kardashians, Khloe cringes when she sees her brother Rob - wearing nothing but briefs - give her BFF a peck on the cheek.

"You guys are so weird together!" she says in the clip above.

She gets disgusted the more she thinks about it.

"So, did you guys have sex?" she finally asks before.

Her pal's answer causes her to let out a huge shriek.

Watch Keeping Up With the Kardashians Sunday at 10 p.m. (EST) on E!