Larry slept with Shawn's sister...

Shannon - a voice-over actress and model - claimed she had an affair with Larry, telling a close pal: “I had sex with Larry, and he bought me an expensive car.”

The car triggered an explosive confrontation between the two women in early July, sources say.

“To help her sister out, Shawn sold Shannon her Mercedes-Benz at a huge discount,” a close friend of Shawn told The Enquirer.

“Just weeks later, Shawn saw Shannon driving a $160,000 car. When Shawn confronted her, Shannon said she’d traded in Shawn’s Mercedes and bought the new car with money she’d earned from voice-over work for a movie. When Shawn asked which movie it was, Shannon claimed she couldn’t remember.”

A suspicious Shawn eventually learned her husband… actually bought the car for Shannon, said her close friend.

“Shawn said she also discovered that Larry had spent $1.5 million to put up Shannon at the swank Beverly Hills Hotel for 18 months,” divulged the friend - who passed a polygraph test. “Shawn was furious! She said that she wouldn’t have felt so crazy if she’d caught Larry fooling around with a stranger. The fact that he’s cheating with her sister has made it so much worse.”…

Shannon’s story also triggered Shawn’s affair with their son’s baseball coach… sources say.