Jimi Westbrook nods vigorously when asked if he was at all worried when he and Karen Fairchild first decided that they might actually be more than just bandmates.

The couple, who form half of the country band Little Big Town, started their music career in 1999 together with Kimberly Schlapman and Phillip Sweet, but it wasn’t until years later that Westbrook and Fairchild got together while single.

“You think about things like that. You wonder, ‘How is this going to change?'” Westbrook recalls, wondering how the relationship upgrade would affect the band.

Eighteen years later, “we’re still trying to figure it out,” he jokes.

However, according to Sweet and Schlapman, the budding romance between Fairchild and Westbrook actually turned out to be good for the band.

“You should have seen them fighting before they got together!” says Sweet. “That’s why it’s obviously better.”

Schlapman agrees with her friend and says “no.” Neither she nor Sweet ever worried about the band’s fate.

“We knew they were in love before they knew they were in love,” she says with a grin.

“We were like, ‘They just have to do it,'” Sweet adds. (Kimberly and I) looked at each other and thought, ‘They like each other!'”

Fairchild and Westbrook married in May 2006, giving new meaning to the word “family” to a band that already felt like one.

In March 2010, their son Elijah was born, along with Schlapman’s daughter Daisy and Sweet’s daughter Penelopi Jane.

In 2017, Schlapman and her husband Stephen adopted their daughter Dolly, completing their family and the band’s offspring.

The band has also gained a number of friends and collaborators over the years.

Her circle includes everyone from Blake Shelton and Luke Bryan to Miranda Lambert and Kelsea Ballerini – the latter both on the Biggest hits Album.

“I don’t think we’ve ever been featured as singers on any of our records,” says Fairchild. “We’ve sung on so many records that we’ve never done it ourselves. So it’s been really great to be able to do that now.”

Lambert, who joins the band in Little White Church, “feels like a fifth member anyway,” says Schlapman. “We had so much fun doing Bandwagon tours with her, and every time we get to sing with her it’s a huge honor. It just felt right.”

Ballerini, in turn, lends her voice to her favorite song of the band: “Shut Up Train.”

“She has loved this song for many years. We talked about Biggest hitsand then I was writing with her and I was like, ‘Well, you should sing ‘Shut Up Train’ with us. That would be great,'” Fairchild recalls. “(Kelsea) was like, ‘Done. Let’s do it.'”

The band’s 2014 hit, “Girl Crush,” is also on the Hits album. The tune remains one of her best known songs to this day, although its radio debut caused controversy as some interpreted the lyrics as a matter of same-sex attraction.

“We played it to Blake in the locker room. The voiceand he stood up and said, “Touchdown!” He said, “I’ve never heard it like that before,” Fairchild recalls of the affirmation the band received from their friends in the industry at the time. “That was obviously very encouraging because we thought, ‘OK, Blake knows what he’s doing.’ I mean, most of the time he does.”

“In music, maybe not in life. I’m just kidding,” Westbrook says, laughing.

“No, we love him. But it was so encouraging to hear him just say that,” Fairchild says. “A lot of our friends felt that way, like, ‘OK, the fans need to hear this.'”

For more on Little Big Town, pick up the latest issue of PEOPLE, on newsstands everywhere now.