Playboy Playmate Confirms Hugh Hefner Tried to Force Her and Her Twin to Have Sex

Karissa Shannon is sounding off on the disturbing claim Hugh Hefner's widow made ... confirming it's "true."

In a new interview with Page Six, the former Playboy Playmate said Hef tried to make her and her twin, Kristina, perform incestuous acts on each other.

She said ... "He would try and force me and my sister to have sex with each other." The twins lived at the Playboy mansion from when they were 18 to 20 years old.

Karissa says that despite Hugh's efforts, she and Kristina "never" actually hooked up with each other, but instead were forced to fake it.

She said ... "Like, really, we never, never had incest[uous] sex. Like, we never did that. We faked it. We faked it every time. And it was horrible. Like, we faked it completely."

Karissa wanted to clear the air after Crystal Hefner accused the Playboy founder of trying to shove the twins' heads "onto each other’s vaginas."

She says Crystal was often there when Hugh was trying to force the sisters to hook up with each other.

According to Karissa, Crystal knew they were faking ... but Hef didn't. She said ... "He wanted to see it and wanted us to fool around with each other and said that if we really loved each other, we would do it together."