It would’ve been the “perfect murder” — but then two sisters’ secret about their father’s death unraveled when they became sexually involved with the same man, according to authorities.

Mary-Beth Tomaselli, 63, and Linda Roberts, 61, were arrested Tuesday for the murder of their 85-year-old father, Anthony Tomaselli, who officials say they initially believed died from natural causes in 2015 at his home in Palm Harbor, Florida.

Pinellas County Sheriff Bob Gualtieri said Tuesday at a news conference the man both sisters were intimate with notified the agency last month that one of the women confessed to the sinister scheme.

The lover — whose identity wasn’t released — met Mary-Beth Tomaselli at a bar in August and they engaged in a sexual relationship. She later introduced him to her younger sister, whom he also became romantically involved with, authorities said.

It was Roberts, the younger sister, who eventually confided in him that she plotted with her sister to euthanize their father, the sheriff said.

The man recorded the conversation on his cellphone as Roberts spilled what unfolded on March 5, 2015.

She claimed the sisters knew their father might die soon and he insisted he not be placed in an assisted living facility, according to the sheriff.

Instead, they allegedly plotted to give their father a sleeping pill mixed with alcohol.

But the scheme didn’t go off without a hitch, officials said. The alcohol allegedly diluted the pills and didn’t kill him.

The women then attempted to suffocate their dad, but when that didn’t work, they stuck a rag down his throat, according to the sheriff.

“It was the ‘perfect’ murder,” Gualtieri said Tuesday.

Emergency responders were called to the home the following day and pronounced the man dead.

The sisters told police they couldn’t wake their father from his sleep. No autopsy was ordered because of their father’s age and medical history, authorities said.

“They clearly knew what they were doing,” Gualtieri said.

Detectives interviewed the sisters, who allegedly confessed to their father’s murder four years ago. Both were charged with first-degree murder and booked into Pinellas County jail.