From PEOPLE Magazine Click to enlarge
My voice is goin'," Caleb Followill drawls softly as he downs his second double-shot margarita during an interview six hours before Kings of Leon's July 29 outdoor show in Dallas, where it's 100 in the shade. "Performing in this heat," says his drummer brother Nathan, who nurses a beer, "is like sucking on a blow-dryer." Or, it turns out, lighting a match: Mid-concert that night Caleb, the lead singer, rants to the crowd about vomiting and having a beer, then vanishes, leaving the band to face the angry audience without him.

The fallout came fast: Bassist Jared told fans to "hate Caleb, not us," then hinted at "internal sicknesses" within the Grammy-winning group. The Kings canceled their U.S. tour, as well as PEOPLE's photo shoot the next day, citing Caleb's "vocal issues and exhaustion." But dysfunction is nothing new for the Followills-brothers Nathan, 32; Caleb, 29; and Jared, 24; and cousin Matthew, 26-who all spoke to PEOPLE the day of the Dallas debacle. In fact, they've captured their alcohol and drug use and family drama in their new documentaryTalihina Sky: The Story of Kings of Leon (Aug. 21 on Showtime). In one scene Nathan screams at Caleb, "You get drunk and you talk s--- ... Your band cannot stand you!" Says Caleb: "You see us at our worst, fighting and drunk. But we wanted to show the good, the bad and the ugly."

That rock-and-roll life-and the success brought by hits like "Sex On Fire"-couldn't be more different from their childhood. Nathan, Caleb and Jared sang hymns at revivals led by dad Ivan Leon Followill, a United Pentecostal preacher, traveling to churches across the South with mom Betty Ann. Homeschooled, they faced strict rules (no movies, TV or secular music), made few friends and shared a handful of clothes. "We didn't think, 'We're dirt poor,'" says Nathan. "Struggles came with our dad doing the Lord's work. We thought we'd have more stars in our crown when we got to heaven."

Their mind-set changed when their parents divorced in the late '90s amidst Ivan's battles with alcohol. "The divorce shattered our world," says Nathan. "We saw that preachers, people we held in the highest regard, fail like everyone else. We realized we won't go to hell if we have some beers." Adds Jared: "When you're super-religious, a level of fear is instilled in you. But I grew out of it."

Not long after, the Followills formed the band, and, while Ivan, 54, and Betty Ann, 51, express discomfort with their sons' lifestyle in the film, they were at the Dallas show, with 25 other relatives. And for all his rock-star rebellion, Caleb is grateful for his upbringing: "I thought I'd be a preacher-now I am, it's just a different audience. Our childhood made us the closest family ever."

Close, but complicated: Nathan and Caleb have brawled over everything from pool to shuffleboard. One night of drinking in 2007 ended with Nathan's dislocating Caleb's shoulder and stabbing his bed with a knife. Though Nathan says neither has thrown a punch in four years, Caleb counters, "I got a bottle thrown at my head two months ago." Retorts Nathan: "That's not a punch." Caleb shrugs off tensions, saying, "I'd fight anyone who touches him. We love each other like that."

Separate tour buses help keep the peace, as does time with their own families in Nashville. Nathan wed singer Jessie Baylin, 27, in 2009; Matthew and wife Johanna Bennett, 26, had a son, Knox, in April; and in May, Caleb married Victoria's Secret model Lily Aldridge, 25. Says Jared, who's single: "Now we're excited to see everyone on the road."

In the wake of the nixed tour, some wonder if Caleb pushed his family-and himself-too far. But after the initial public venting, the band has been united, with Nathan tweeting about Caleb's month of doctor-ordered vocal rest, and Jared posting, "Family has to come first," and dismissing talk that they're urging him to go to rehab. As for the frontman under fire? "I'm a damn sinner in every way," says Caleb. "All of us are. But I'm doing the best I can."