After Rain Phoenix’s brother River died in front of her in 1993, having overdosed on Sunset Boulevard as she and their younger brother Joaquin looked on helplessly, she disappeared into the shadows. This being the Phoenix family, her concept of disappearing didn’t involve just hiding out in her bedroom for the next few years. Instead Rain, then barely in her 20s, went on tour as a backing singer with some of River’s musical friends. These friends happened to be the Red Hot Chili Peppers and REM. The latter were touring their album Monster, which they dedicated to River.

She also, tentatively, began to go through the archive of Aleka’s Attic, her brother’s band, which she had joined at 16. When she got back to Los Angeles after touring, she moved into the guesthouse of some other friends of River’s, actors Catherine Keener and Dermot Mulroney, and spent 1996 and 97 mixing all of her brother’s unmixed songs. “And that,” she says, “was hugely healing – just spending that time listening to my brother. After I finished, I didn’t feel the need to release the music. It was enough.”

That was two decades ago and things have changed. “I’m an adult now. My brain was not fully formed at 20, and you process things differently. Now I feel a sense of joy to share my brother with others. But it’s taken a long time to get here.” Earlier this year, Rain, 46, released two previously unheard Aleka’s Attic songs, Where I’d Gone and Scales & Fishnails, and she is planning more. “The project came together so gracefully,” she says. “Then I started to make my own music.”

‘I couldn’t put out a solo album without him’ … Rain and River performing in New York in 1991. Photograph: Steve Eichner/Getty

She made so much music that it turned into her first solo album. Out today – the 26th anniversary of River’s death – it’s a darkly romantic meditation on loss that wonders if those who die are gone or live for ever through us. “Your light, I keep it alive,” she sings on the album’s best song, Immolate, which she released on River’s birthday: 23 August. The lyrics of another, Lost in Motion, were largely written by him. She has called the album, simply, River. “As I was writing the music, he kept coming to my mind and it suddenly came to me that I couldn’t put out a solo album without him, so that’s why I called it River.”

River’s friends have also contributed: Gus Van Sant, who directed him in My Own Private Idaho and Rain in Even Cowgirls Get the Blues, shot the video for Immolate. Keener and her son directed the one for Lost in Motion and Michael Stipe duets with Rain on Time Is a Killer. She calls Stipe “an older brother to me. I often send him my songs to get his opinion. His generosity and strong ethical framework – they really mirror the way River was.” She speaks fondly of Flea from the Chili Peppers, too, and spoke to Keanu Reeves only recently. By keeping her brother’s friends close, Rain has found a way of keeping his light alive.

An exclusive first view of Lost in Motion, with lyrics by River

The Phoenix family have always been very careful in maintaining their privacy, rarely speaking to reporters about River. Doesn’t this album and the release of Aleka’s Attic songs mean she now has the burden of answering questions about her brother from journalists like me? “It’s not a burden,” she says. “Celebrating River now, it just feels joyful.”

But why did it take a quarter of a century to get to this place? Or, to put it another way, why now? She is speaking by phone from LA and there is silence at the end of the line. After a full 30 seconds, I ask if she’s still there. “Oh yes, I’m just thinking about what I want to say,” she says, in a tone that suggests she is not one to be hurried. “I’m so grateful for my self-preservation at the time of his passing. I respect how I processed it, and I believe that grief, being its own wild animal, does not choose a finite amount of time for us to process it. It’s something that will continue over my lifetime. But I am able now to articulate what my brother River was to me in a way that does him justice.”

Gunslingers … with Uma Thurman in the 1993 film Even Cowgirls Get the Blues. Photograph: Fine Line Features/Kobal/REX/Shutterstock

She is very careful about what she articulates, though, and assiduous in maintaining her boundaries. Specific questions about her childhood are met with a short: “I don’t see what that has to do with the album.” When I ask about her father, who died recently, she interrupts, saying she will respect his lifelong wish for privacy and not discuss him. Queries about how her family coped after the death of River are met with similarly short shrift: “I will not answer for them.” She doesn’t come across as rude; instead, there is a pleasingly no-nonsense attitude to her that is both charmingly naive and impressively self-assured. When I ask if she has children, Rain simply says: “I’m a very devoted aunt, but I feel like I was put on this Earth for things other than motherhood.”

With deep-set eyes and dark colouring, Rain looks strikingly like her youngest brother, Joaquin, as opposed to the blond, angelic River. But as the two eldest of the five siblings, she and River were in many ways closest to one another. As soon as she says this, she adds: “We were all close. There was a lot of laughter in our group. Laughing. That’s what I think of when I think of us as children.”

The Phoenix kids had a notoriously unconventional childhood. In 1972, their parents, John and Heart, joined the Children of God, which they believed was a Christian missionary group, and the family travelled around South America spreading its word. By 1976, however, she says they had realised the Children of God was actually a cult. “My parents stole away into the night with $100 and found a cargo ship that would take us to Florida,” she says. “It was decades later that we learned just how disturbing the organisation had progressively grown to be. I’m so grateful my parents had the foresight to get out when they did.”

‘Wonderful’ … Rain and Joaquin, who came to her recent record launch fresh from the Joker premiere. Photograph: Matt Winkelmeyer/Getty Images

Back in the US, Rain and River, at the ages of six and eight, occasionally sang on street corners for money. “I liked it!” she smiles. “That’s how we rehearsed our songs.” They both also worked as child actors. Whatever has been said about their unorthodox background, the family remain extremely close: all her siblings and her mother came to the small concert she recently gave to launch her album, with Joaquin, fresh from the Joker premiere, loudly cheering her from the audience. Singing River’s lyrics in front of her family was, she says, “wonderful”.

River became extraordinarily famous extraordinarily young, but the entertainment business, with its interest in commerce over creativity, was completely at odds with the free-spirited world he grew up in. “There is a part of River that I carry in me, this extreme left-of-centre rebelliousness, thinking outside the box, doing whatever you have to do to let your soul speak.” When the media snarked about Aleka’s Attic back in the 1990s, Rain was aware of the “poo-pooing” but says River was “unapologetic, and that has been the guiding light of my career”.

As well as his talent and beauty, River was famous for seizing any opportunity to discuss vegetarianism and the environment with often bemused entertainment reporters. It is not hard to draw a line from the evangelising he did as a child on behalf of his parents and the cult, and the activism he took on as an adult. Last year, I interviewed Samantha Mathis, River’s girlfriend at the time of his death, who was also with him when he died. She told me that, while River loved the work and the opportunities to talk about his beliefs, he hated the “industry bullshit”, and his relationship with success was complicated.

Rain agrees. “That’s true – but he had a sense of humour about it. He wasn’t a dick about it! His former publicist, who’s now Joaquin’s publicist, called me up laughing the other day to tell me that she’d just remembered how, when she’d call River to tell him what the press were saying about him and he’d go, ‘You’re breaking up!’ And he would hang up,” she says with a laugh.

So was that how she learned how to maintain boundaries and protect herself? “Yes and no. I think some of it comes with age. When I hear a question I think, ‘What do I want to come through here?’ And yes, River was like that. Each of us has a seed of gentle truth in us, and I’d rather be inspiring others than feeding into sensationalism and fame. There are all kinds of things that would make great headlines, but that’s not River and it’s not me. I just want to be creating a groove of our true being, like River would have done. He always was the guiding light for me on this plane, and he still is now.”

• Rain Phoenix’s album River is out today.

This article was amended on 1 November 2019 to reflect the fact that Rain and River busked in the US, not South America.