“Our dad is in London right now, building the spider that we’re taking on tour,” singer-songwriter Finneas O’Connell tells Billboard, in a tone like he’s reading off a grocery list.

That “spider” is in reality a giant light rig made to look like a tarantula, while the “we” is O’Connell and his younger sister -- the teenage dark-pop sensation Billie Eilish. They’ll kick off a five-week tour across Europe tomorrow to drum up excitement for Billie’s debut album out March 29, When We All Fall Asleep, Where Do We Go?, which O’Connell co-wrote and produced.

In other words: Welcome to the family.

Raised by actor-musician parents, O’Connell, 21, and Eilish (who goes by one of her middle names), 17, have become a formidable team in the music industry. From their childhood home in Los Angeles, O’Connell co-wrote and produced his sister’s haunting breakthrough “Ocean Eyes,” which went viral on SoundCloud in 2015. He did the same for Eilish’s lauded EP dont smile at me two years later, and has joined her backing band on worldwide treks and festival dates since. The sibling duo have landed eight of Billie’s tracks on the Hot 100; the most successful to date, album lead single “bury a friend,” peaked at No. 14. In April, they’ll perform at Coachella for the first time before embarking on a world tour to promote Eilish’s album in May.

Visiting Billboard’s New York offices, O’Connell instinctually takes a seat at the lone piano in the room, pulling back the sleeves of his grey sweater to reveal an Apple watch with a Gucci strap. He’s wearing large pink sunglasses, which he fiddles with while describing the bond he has with his sister as “unshakeable.” “I have a house now, and don’t live at home with the family anymore,” he says. “So the thing I've been the most excited about for this tour is just to hang out with Billie for five weeks straight.”

Today, though, O’Connell is riding solo -- something he’s been doing more often lately, as he builds his own career as an artist. He’s released a string of unabashedly romantic indie-pop songs simply as Finneas, beginning with 2016’s steamy “New Girl” and most recently, the pining “Claudia,” about his current girlfriend. The night prior, he performed his first solo New York headlining show (and second headlining show ever) at hip Brooklyn haunt Baby’s All Right. Meanwhile, he’s quietly widened his circle of collaborators to include dream-pop singer Ashe, along with R&B upstart Sabrina Claudio and British crooner Bruno Major on their forthcoming projects.

“The venue was the first New York headliner my sister played, and I played in her band for that,” O’Connell says about the packed show at Baby’s. “To get to come back, sell it out, and have kids sing my songs was a crazy feeling.”

Such an experience has been on O’Connell’s bucket list since he was 11 years old, growing up homeschooled in Highland Park, Los Angeles. The siblings’ parents, Maggie Baird and Patrick O'Connell, are both actors and musicians -- Baird has appeared in shows like Bones and The X-Files, while Patrick has scored parts in The West Wing and Iron Man. Following his mother to many an audition, O’Connell says he got a “realistic” glimpse at the stamina needed to make it in entertainment. “You might think of Hollywood as this full-on glamorous thing, and to us it was like, ‘All right, mom's got an audition,’” he jokes. “'Do you want to sit in traffic for 50 minutes and go in with her?'”

Still, watching his parents navigate the competitive industry only made him work harder. In high school, he formed his first project, alt-rock band The Slightlys. To most teenagers, O’Connell says, starting a band “meant when we feel like it, when it's fun, maybe.” But to him, “it was like, ‘All right, here’s the rehearsal schedule.’” Those efforts paid off: The Slightlys put out three songs, won as many local Battle of the Bands competitions and even played Warped Tour in 2015.

At the same time, O’Connell was taking a shot at acting. He snapped up his first major film role as a scruffy student of Cameron Diaz’s title character in 2011’s Bad Teacher, and earned recurring roles in Modern Family and the sixth season of Glee. He starred with his parents in 2013 family drama Life Inside Out (which his mother co-wrote), playing the troubled teenage son who discovers his musical talents.

Eventually, though, acting began to get in the way of O’Connell’s music goals. He decided to focus on the latter around age 13. “Working on TV shows was fun, but I felt crazy pressured and stressed,” he remembers, describing himself as an anxious kid. “I always wanted to be on tour or making albums.”

But O’Connell says his experiences with taking on characters give him an edge in the studio, where he’s adept at writing from the perspective of his sister and others. When he writes with Eilish, O’Connell says, they write specifically for her vocal range and about her life.

“Being able to hear an artist and emulate them has been a huge part of being successful as a producer and co-writer,” he says. “I think it's a problem when a producer comes in to work with an artist, and you can't hear the artist as well anymore. It’s very important to me to be invisible.”

Eilish’s catalog is scattered with inventive vantage points -- in the smoldering “Bellyache,” she takes on the persona of a conflicted serial killer; in piano melody “idontwannabeyouanymore,” she imagines a conversation with herself. (Both are co-written and co-produced by O’Connell.) It helps that Eilish has a knack for selling any song like it’s her own. “The first time she sang, she covered [Drake’s] ‘Hotline Bling’ on a ukulele in our house,” O’Connell remembers. “I was like, ‘Did you write this?’ I believed it.”

Prior to “Ocean Eyes,” the siblings had done two songs together; one that O’Connell wrote for her, and another that Eilish wrote alone. Originally, O’Connell wrote “Ocean Eyes” for The Slightlys. But when Eilish’s dance teacher suggested the two compose a song for original choreography, the swaying, mid-tempo “Ocean Eyes” made perfect sense. O’Connell taught her the song, and they posted it on SoundCloud with a free download link, mainly so that Eilish’s dance teacher could access it.

What happened next has been well-documented: Within days, O’Connell remembers, the song reached nearly 1,000 plays. Then, it exploded, attracting interest from Interscope, where Eilish signed in 2016, and from Eilish’s now-manager Danny Rukasin. O’Connell remains independent.

By now, “Ocean Eyes” has raked in more than 250 million streams on Spotify, rivaled only by single “when the party’s over” (307 million) and “lovely,” Eilish’s collab with Khalid, which has 440 million. “That’s a scary number,” O’Connell says of the first total, bewildered. “It’s so many people.”

Now, as he builds a steady stream of his own material, O’Connell’s personal sound is taking shape. His songs tend toward lush piano chords and breathy vocals, and they’re often intensely personal. The lyrics of predictably heart-wrenching ballad “Break My Heart Again” are taken nearly word-for-word from a text conversation with an ex, and he shared the song with her before releasing it (it made her sad, but she approved). “I try not to shy away from specificity,” he says.

Speaking of lyrics, his songs often weave in and out of each other, similar to the way Eilish’s “COPYCAT” steals its chord progression from its sonic foil, “idontwannabeyouanymore.” In the swingy “Let’s Fall in Love For the Night,” O’Connell sings, “I like to push my luck,” a wink to his acoustic track “Luck Pusher.” He plans to put out an EP or album eventually, but for now finds releasing singles more satisfying: “I get to put different artwork on all of them, give them each their own moment and tell a story that way.”

With each release, he’s experiencing new attention. While debut single “New Girl” has drawn two million Spotify streams, “Claudia,” released last month, is already more than halfway to matching that number. And “Let’s Fall in Love For the Night,” released last October, is already up to 6.7 million streams. O’Connell likes to humbly joke that his girlfriend, who is well-known on Instagram, gets stopped for photos more often than he does. But at that recent sold-out gig on a rainy Monday night in Williamsburg, a horde of Eilish look-a-likes dressed in streetwear threw their Sharpie-X’d hands in the air, chanting his name.

“Take it off!” a particularly rowdy group yelled in unison, to which O’Connell responded with an eyebrow-raise and a smile. Taking a seat at the piano, he launched into 2017’s “I’m in Love Without You,” an EDM-infused breakup track, and the second single he ever put out. The crowd knew it word-for-word.