Don't mess with the K clan!

Kim Kardashian arrived at Casa Vega in Sherman Oaks this afternoon to grab lunch with sisters Khloe and Kourtney, and rumor has it the ladies are busy plotting to expose brother Rob's sleazy fiancee Blac Chyna as a cheater.

According to Page Six, the former stripper was spotted getting close to a mystery man at a Super Bowl viewing party in LA on Sunday, and the family wants her gone once and for all! Since they know Rob is too blindsided by love to want to know the truth, they're going to find it out for him.

"Rob’s sisters have hired a private investigator to follow Chyna and catch her cheating," a source dished to Radar.

Unfortunately, it's a lose-lose situation, because they know if they find something the sock designer will be crushed.

"It is a double-edged sword. Rob’s sisters know that, if they catch Chyna cheating, it will break Rob’s heart! The last thing that anyone wants is for Rob to slip back into the dark depression he was in before meeting her," the source added.

We don't know what's taken them so long to reach their breaking point! Is there anyone that still believes Chyna isn't out for fame and fortune only?