A Hamptons businessman is being sued by a relative for allegedly raping and sexually abusing her for four years, starting when she was 15.

The suit against Steven Mezynieski, a Southampton excavator and owner of the historic Driftwood Farms in Orient, comes five months after State Police charged him with first-degree rape and first-degree criminal sexual act. He has denied the charges.

According to the suit filed Thursday in state Supreme Court in Suffolk, Mezynieski “groomed” the victim by constantly telling her she was pretty and “special” and by taking her on shopping sprees and trips. His wife, Gretchen Sontag-Mezynieski, was also named in the suit because she allegedly knew what was going on and allowed it to continue.

The relative, 20, seeks unspecified punitive damages and compensation for emotional stress, sexual assault, unlawful imprisonment, conspiracy and other alleged wrongdoing. She is identified in the suit as “Jane Doe.”

“They’re really ridiculous and fantastic charges,” Stephen Scaring, the Garden City attorney representing the couple, said Friday.

He said the Mezynieskis believe their relative is trying to extort money from them by raising “sensational and embarrassing” allegations that are false.

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In April 2011, the teenager began spending more time with the couple in their Orient home, allowing her parents to focus on her brother, who had cancer, said the plaintiff’s attorney, Rosemarie Arnold of Manhattan.

The alleged abuse started with Mezynieski peeking at her trying on a bra in a store at the Smith Haven Mall, according to the suit. Over the years, it progressed from unwanted touching to rape, the suit said.

The abuse happened even in front of Mezynieski’s wife and young children, according to the suit.

Mezynieski owns the 140-acre Driftwood Farms, which board horses and grow privet hedges, as well as an excavation and road-construction business based in Southampton.

According to the suit, he told the teenager no one would believe her about the sexual assaults, and she stayed silent because she didn’t want to add to her parents’ emotional burden.

Last July, she finally told her parents about the alleged abuse, and they reported it to police, according to the suit.

When confronted by the girl’s mother, Mezynieski handed her an envelope containing $15,000, saying he wanted to “make things better,” the suit said.

The young woman has been “disabled” by the alleged abuse, her attorney said.

“She has had to leave college,” Arnold said. “She has nightmares, flashbacks, anxiety, lack of self-worth, depression.”

The criminal case is pending in Southold Justice Court.