For 25 years, Toby Willis ruled over his family of 12 musical kids like a dictator. No one dared disobey the patriarch — seen on TLC’s “The Willis Family” — even though he was also sexually molesting at least four of his young daughters. Other kids were beaten, some to the point of bruises and blood.

Today, Willis, 52, is inmate No. 581910 at the Tennessee state prison in Tiptonville, where he’s serving a 40-year sentence for child rape. His oldest daughter, singer-songwriter Jessica Willis Fisher, bravely reported him to authorities in 2016.

Her new memoir, “Unspeakable” (Thomas Nelson), chronicles both her harrowing childhood in a fundamentalist Christian family and the nightmare that ensued when she tried to escape from the family six years ago. The Willis clan first rose to fame in 2014, when they appeared in season 9 of “America’s Got Talent” and later starred in the short-lived TLC show in 2015 and 2016.

Willis Fisher says she has some good memories of life with her siblings growing up, including frequent tours in Ireland — but the bad far outweighed the good.

“I was literally doing shows, singing and putting on a performance but I was also putting on a performance every day offstage as well pretending to be happy,” she told The Post during a recent trip to New York with her husband, photographer Sean Fisher. “I was spanked and beaten for not smiling.”

Even at 23, despite being the lead singer, fiddle player and main songwriter for the family band, Jessica was not allowed a cellphone, and she was forbidden to go on dates without a chaperone. Like many of her siblings, her clothing and hairstyle had to be approved by her dad — who kept a stash of AR-15s at the house. When she began to rebel and indicated she wanted to leave home in her early 20s, her father called her “demonic.” Her mother and some of her siblings urged her not to go and blamed her for upsetting the family dynamic, though they were well aware of Toby Willis’ long history of abuse.

Toby’s own childhood was marked by tragedy. His parents, pastor Scott and wife Janet, were the only survivors of a fiery van crash in Wisconsin in 1994, in which six of their nine kids were killed. The family later won a $100 million settlement.

A champion high-school wrestler, Toby met his future wife Brenda when they were both teenagers. They married around 1990 and quickly began growing their family; Jessica, the eldest daughter, was born in the Chicago area in 1992. The family grew — there would be 12 kids in total all with names beginning with “J” — and in 2001, they moved just outside of Nashville, Tenn., in a house that once belonged to country star Randy Travis. They homeschooled their children, imposing strict extracurricular activities including wrestling, competitive swing dancing, playing instruments and singing.

After remaining silent for decades, Willis Fisher, now 30, first wrote about her childhood in a shocking 2018 blog post that began in part, “As far back as I can remember, I was sexually abused by my father. I figured out I was about three years old in some of the earliest memories.”

Her mother approached her when Jessica was nine and asked her if her father was behaving inappropriately. Jessica nodded — a bit uncertainly. But her father denied everything and then became more surreptitious and “dangerous” in his abuse, Willis Fisher writes. “I would say it’s astounding how it wasn’t talked about,” she says of the years at the Willis home and on tour when she and her sisters were molested. “Much later, we got to the point where there were conversations about the problem, Dad’s problem. But none of those times did I say that Dad this or that to me.”

It did not come out until August 2015 when Willis Fisher, in agony and a deep depression, wrote a 14-page letter to her mother, pouring out the often “graphic” abuse she suffered at the hands of her father.

Despite the shocking revelation, nothing really changed in the family where Toby enjoyed cult leader-like status.

“My mother entreated me to conform during (standoffs with her father), proving her sympathy was only available if my father was not there demanding her support,” writes Willis Fisher. “When I pulled her one way and Dad the other, Dad’s strings always won.”

Brenda Willis, now 53, lives outside Nashville with her younger kids. Six of the Willis children, including Jessica, have married since their father’s imprisonment in 2016. Jessica said her mother has divorced Toby, although she kept a photo of him on her Facebook profile for months after his arrest before finally deleting it.

“I know my mother loves me,” says Willis Fisher. “But yes, she let me down.”

Brenda Willis declined to comment and abruptly hung up when The Post called her. But she has steadfastly backed Jessica on her Facebook page, linking to her memoir and saying that she is proud of her.

With the encouragement of her now-husband Sean, the son of Pete Fisher, who ran the Grand Ole Opry for 30 years before stepping down in 2017, Willis Fisher finally gathered the courage to leave her family home on April 7, 2016, just hours after her father beat her with a belt and demanded she apologize to him after an argument.

She writes about being physically prevented by her family from leaving; her baby sister sobbed and held on to her legs. But on April 7, something inside her made her snap. She was determined to leave home for good.

“Staying helps no one,” Willis Fisher writes of her realization. “I cannot save them here. I thought I might die if I didn’t leave.”

She later contemplated reaching out to her father in prison and possibly visiting him, but when she found out he had the option of turning down the request, she says she didn’t want to give him that choice.

“The truth is I don’t know what was going on in my dad’s head,” she says. “Did he really think we were going to go on like that forever? Our family was like a cult and there are cults that go on for generations. Is that what he was hoping for? I don’t know. It’s chilling to remember some of the dreams he had for us and his grandchildren.”

Despite everything, she remains loyal to her mother and siblings, and stresses that the book is her own personal experience — she can’t speak for her whole family. “Unspeakable” is not overly graphic in its depiction of the abuse Jessica suffered, but it does not shy away from some horrific details and comes with a trigger warning. Willis Fisher also released a well-reviewed album, “Brand New Day,” in April. She wrote nine of the 10 songs and says it serves as a complement to her memoir, which she calls her “witness statement.”

“Initially, I wrote this book for my own healing,” Fisher says.

“Whether anyone else saw it or not, that was the first and most important thing. Because for almost 25 years I was trying to make other people happy. I was trying to protect them whether it was as performing in the band or trying to put on this show that everything was okay, but inside I was suffering. So the priorities had to get reordered immediately and I had to start taking care of myself.”

Today, Fisher Willis lives with her husband in Nashville and is friendly with her siblings and mom. She performed this past year at the Grand Ole Opry, where she appeared with The Willis Clan as a teenager, and appears regularly at other venues.

“I don’t want to be defined by like what my dad did,” she says. “I’ve had people ask me,  ‘Do you want to be defined by this?’ — But since I don’t define myself by this, it’s not my problem if others do.”