WHEN Allison Moorer was 14 and her sister, Shelby Lynne, was 17, their estranged father, Vernon, turned up at their home. Outside the house, he and the girls' mother became involved in an argument. The girls were cowering inside when Vernon shot their mother dead and turned the gun on himself, committing suicide.

It’s the kind of horrifying loss that, as Moorer herself says, some teenagers might not have survived. But Moorer and Lynne did more than survive. They went on to become huge names on the country music scene, with their own, distinctive and stellar solo careers.

Some 31 years on, they have created an album together, fusing their two voices, and their story, for the first time. On Saturday, they will perform songs from this collection, titled Not Dark Yet, at Celtic Connections. Listening to their voices, it's hard not to hear hints of what happened back on that tragic day in 1986.

“We were very young when we lost our parents,” says Moorer, 45, as she speaks from New York, where she part-lives, dividing her time between there and Nashville. “That happening affected us in a lot of different ways. And living with our parents before they died affected us in a lot of ways. They showed us how to be artists. They exposed us to music. But they also showed us a lot of horrible, terrible things which we both lived with. And that’s everyone’s life. You take the best that you can out of whatever experience you have.”

Of course the songs on the album – many of them cover versions – are not all dark, just as not all of the tales Moorer tells are tragic. Many of her anecdotes about her rural Alabama childhood make it sound almost idyllic. The girls grew up surrounded by countryside, books and music. “We also,” she recalls, “knew how to go get a fishing pole and go down to the pond and catch fish, or how to tie a knot.”

But the shadow is there in the album, particularly in their co-written single, Is It Too Much? “No-one else walks upon this road,” the lyrics declare. “No-one else bears this heavy load/ Is it too much to carry in your heart?”

Was this a line she and her sister have often wanted to say to each other? “It’s one thing,” Moorer says, as if to indicate there are many others. “We’re very close. We’re very bonded as any sisters are. We’re bonded musically. We also have the bonds of family, the bonds of trauma. There are certain things about our lives that nobody understands but the other. So yeah I think there’s an element of that. We tend to be very open with each other and very supportive in helping the other through whatever heaviness we’re carrying about.”

Moorer has been married twice, both times to musicians, first Doyle Lee Primm and more recently, singer-songwriter Steve Earle, who she divorced in 2014. Earle is also father of her child, John Henry, who is autistic and non-verbal. Moorer is reluctant to talk about her ex-husband save to say that he had no influence on her music. “The last time I toured with Steve I played in his band for free. If you want to print that you can, but otherwise I don’t want to talk about Steve.”

However, Earle has himself been vocal in the press about being “dumped” by Moorer. When asked by Guardian writer Simon Hattenstone, if their son’s autism contributed to their split, he said: “I think it was the straw that broke the camel’s back, but I think she was going to leave me anyway. She traded me in for a younger, skinnier, less-talented singer-songwriter.”

Not long after her son, John Henry, was born seven years ago, Moorer began writing a memoir of her own childhood. “I realised that he would not know who my parents were if I didn’t write it down, if I did not figure out some way to tell him the story, besides anecdotally.”

She also wanted to ensure John Henry didn’t have to rely on the media versions of his family story as history. “Anyone can say anything and it can get printed. I think it’s important to me that my son know as much about his grandparents as he possibly can and I thought that the best way to get that down was to write a memoir about that period of my life.”

As she has gone back over memories, she says, she has been “trying to figure out a way to wrap language around the story”. “That has made me look at what really happened in our family, possible reasons for what happened. Mainly I’ve been able to find a lot of empathy for my parents.”

It’s not the first time she, or her sister, have looked at their father’s story with some empathy. Back in 2011 Lynne wrote Heaven’s Only Days Down The Road, a song which essentially told the story of that fateful night in 1986, but from her tormented, alcoholic father’s point of view. “Hundred so miles from the Moblie River/Lord I can’t have her so I got to kill her,” the song begins. “Been insane since I was nine,” one lyric declares. And then a few more lines further on come the horrifying lyrics: “Can’t blame the whiskey or my Mammy’s ways/ Two little girls are better off this way.”

“I don’t think anyone can look at a story like his,” says Moorer, “and not feel some empathy, regardless of what your relationship is. He was obviously a person who was in a tremendous amount of pain.”

Did she love her father? “Well, of course, I loved him. And they were loving, talented people, my parents, and of course they’ve been reduced to these caricatures in the press. That’s bothersome to me.”

One of the things she came to realise was that she “really hardly knew” her parents. “I do not know nearly enough about them as people. When you’re a kid you think your parents exist only to be your parents. And what I found out, and what I think most people find out, is that your parents are people before you’re born and they’re people after you’re born. That’s not all there is to a person – just being your mother or your father.”

Far from dwelling on the negatives, she frequently describes her parents in a positive light. “As messed up as my childhood was, my parents were really good parents. They gave us boundaries. They also let us explore. And when it comes to knowing how to navigate the world, we both do all right. They did a pretty amazing job in the short time that they had. I think a lot of people given our certain circumstances probably wouldn’t have lived through it. I have to tip my hat somewhere. And it’s mostly to our mother. We had a really good mother.”

Their mother, Laura Lynne, taught her daughters how to act in the world. “She knew how to conduct herself. She had a beautiful comportment. She had manners she had grace, and I can’t give her enough credit for really instilling those values in us.” She pauses then adds: “That’s without getting into all the psychological issues that living with them and all their addictions and sicknesses and how they went out, passed down to us.”

Vernon Franklin Moorer’s addiction was alcohol. What was their mother’s? “I think her addiction was to him,” says Moorer. It’s been reported that she tried to leave her husband many times. “I don't remember how many times,” Moorer says, “You know, nothing was ever stable. That has a deep effect on a child.”

Moorer cannot remember a time when she did not sing with her sister. “We were just surrounded by music and so it was something we considered a very natural part of life. We started singing publicly when we were very young.”

Their father played the guitar. But it was their mother, she recalls, who really taught them how to sing. “She sang harmonies while she was cooking dinner. We lived in a really rural area so we would drive every morning to where she worked and we went to school, which was about 30 miles away. And we had no working radio, so we would sing. My sister and I learned to sing in that car. Our mother would teach us all songs and the three of us would harmonise.”

It’s not hard to see why Lynne and Moorer's musical relationship is so intense. It reflects their bond. Moorer’s relationship with her sister, who became her caretaker when her parents died, has dominated her life, seen her through everything. Sometimes Moorer refers to Lynne as “the other”. "Walking through the world,” she says, “with only the other as real, immediate family, has made us very close." Singing together, meanwhile, "is like falling off a log".

“It makes total sense. It feels like home for both of us, it feels like going home. Our voices blend very naturally. We don’t really have to find our place with the other. We’ve been doing it so long we know.”

The album Not Dark Yet is available now. Shelby Lynne and Allison Moorer will be performing with Teddy Thompson at Celtic Connections on Saturday, January 27 at 8pm. The Sunday Herald is the festival's media partner www.celticconnections.com