That was fast! Just weeks after the Kardashian family welcomed pregnantBlac Chyna into their inner circle, RobKardashian’s sisters have turned against her, we have learned, amid claims that the only male Kardashian might not be her baby’s father!

As Radar reported, Blac Chyna and singer Pilot Jones have been running around behind Kardashian’s back for months. After viewing a series of photos that showed the 28-year-old former stripper and Pilot kissing and caressing each other, an insider said, “When that baby comes out, it’s going to be brown! The truth will come out eventually. Pilot is the dad.”

Indeed, in March – when Blac Chyna and Kardashian’s relationship hit a rough patch – she snuck out of L.A. to Atlanta to be with Jones, evidenced by photos on Radar that showed the two dancing very close at Medusa nightclub!”

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“Rob will absolutely request a DNA test after all this,” a Kardashian family insider said. “That was going to happen even before all of this mess got out there.”

However, as fans know, Rob’s superstar sisters – Kim, 35, Kourtney, 37, and Khloe,32 – are fiercely protective of their brother. And now, according to the source, they have already contacted a private investigator to uncover the truth about Blac Chyna’s baby daddy.