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He was there with her on stage in front of 50,000 people at Coachella in April. He was there throughout the writing and making of her debut album, one of the year’s biggest releases. He was there when she sang his song “Ocean Eyes” for the first time, before it racked up 300 million streams.

While it was great sharing those accomplishments, Finneas O’Connell wishes he and his little sister Billie Eilish could do far more mundane activities together nowadays.

“I really value just being able to go out and grab a coffee or going to a movie and not have anyone recognize me,” said the singer, songwriter, bandmate and producer who performs simply as Finneas. “Billie is basically too famous to do any of that at this point. She has to have security with her just to go see a band she likes or whatever.

“It’s wonderful our music is doing so well and people are into it, but it has definitely altered her private life in a way I don’t envy.”

Just 17 years old, Billie Eilish O’Connell — who uses her middle name as her stage surname — has turned into one of pop music’s biggest newcomers of the decade. Tickets sold out instantly for her concert next weekend in Minneapolis.

Her full-length collection of jaggedly rhythmic, candidly intimate stream-of-consciousness electro-pop, “When We All Fall Asleep, Where Do We Go?” has remained at or near the top of the Billboard album chart for two months now. She’s the first artist born in the 21st century to land a No. 1 record. She also broke the record for having the most songs in the Billboard Hot 100 in the same week (12).

Perhaps an even greater signifier in 21st-century terms, her songs have been streamed 4 billion times just in the U.S., going back to when “Ocean Eyes” took off via SoundCloud in 2015 — a song her brother wrote for his high school band.

Four years Billie’s elder, Finneas has been her producer and only songwriting partner throughout her fast ascent. And now he’s one of two band members performing with her on the tour that lands next Saturday at the Armory in downtown Minneapolis.

Calling last month from his newly acquired house in Los Angeles’ Highland Park neighborhood — very near the house where Billie still lives with their parents and all their music was made — Finneas simultaneously marveled at and shrugged off the impressive numbers their work has attained.

“It’s shocking and very overwhelming,” he said. “I feel like I have a handle on it now, but in a few years I bet I’ll look back on this time and say, ‘Omigod, I didn’t know what … was going on.’ ”

As for his sister, he said Billie is holding up remarkably well under the demands of her newfound fame. Both parents usually join them on the road, so Finneas said he gets to play the role of friend and bandmate more than protective big brother, which maybe isn’t needed anyway.

“I really care about her and worry about her whenever she has a lot to do, as any family member would,” he said. “But she’s kind of a badass. It’s not like she needs help taking care of herself.”

Billie was 14 when she recorded the ethereal ballad “Ocean Eyes” in the home studio Finneas kept in his bedroom. She wasn’t dreaming of it making her famous. She simply recorded it at the request of her dance instructor, who asked for an original song to accompany some new choreography. They uploaded the song to SoundCloud simply as an easy way for the teacher to access it.

The O’Connell kids were both home-schooled by their semiprofessional actor parents, who encouraged all their creative pursuits.

Finneas, too, has done a bit of acting, most notably with small but recurring roles on the TV shows “Glee” and “Modern Family.”

Even before “Ocean Eyes” set off a tidal wave of interest, Finneas had turned to music-making as his role of choice.

He has been releasing his own singles for three years now, drumming up his own streaming numbers with the John Legend-style romantic piano ballad “Break My Heart Again,” the sexy pop jam “New Girl” and the Sam Smith-dramatic “I Lost a Friend.” Another of his most popular tunes, “Claudia,” was written after he met his girlfriend, YouTube vlogger Claudia Sulewski.

Finneas performed as the opening act on many of Billie’s overseas live gigs. On her current U.S. trek, however, he opted to stick to playing keyboards and guitars alongside drummer Andrew Marshall.

“It’s nice to sing a little bit of my music for her audiences, because they’re so nice, but it’s also exhausting,” he said. “Performing in her band is also a joy, so I’m just focusing on that this summer.”

Here’s more of what Finneas had to say about his and his sister’s rapidly evolving stories.

Why their working relationship works so well: “We’re very close. Being home-schooled, we spent a lot of time together growing up. So we know each other well.

“People always say, ‘I couldn’t do what you do, because my siblings and I get into fights.’ Well, so do we, but it’s actually awesome. We’re not afraid to tell each other what we think. I think because we’re siblings we’re more comfortable with the songwriting process, and working to make songs better if the other one doesn’t like what we’re doing. We’re more honest.”

On their songwriting process: “It’s different every time. Sometimes it’s her life experience that sparks it; sometimes it’s mine. Sometimes one of us will do a bunch of the writing and then the other comes in and helps finish it.

“A good example of that is ‘You Should See Me in a Crown.’ I had mostly written it, but when it came time to record it she had some good ideas for changing up some lyrics. But then ‘I Love You,’ from the word go we were in the room together writing that one and every line we came up with together.”

Why they continue to record in bedrooms instead of studios: “If you need to record live instruments, especially drums, it’s still best to do it in a studio. But anyone producing themselves, that’s all going on in bedrooms these days, and I’m all for it. I feel like you’re able to be your most creative in private environments, and not a studio where an A&R person is coming in, telling us a song isn’t a smash.

“We feel really happy and more inspired when we’re at home. Also, daylight helps. My bedroom had a lot of windows, and we found that to be very important.”

On recording in his new house: “I found a place in October and bought it. I sort of just rebuilt everything I have at the family house here, and left everything there. So if we do wind up working over there, my stuff is still all set up. We’ve not had a ton of time to record together, but I have done some recording here on my own and with some other artists, too. I’ve learned a lot from working with new people.”

On balancing his solo career with his sister’s: “Hopefully, I’ll get to do both as much as I can. It helps that we have the same team — same management, same booking agent — so I make sure they consider both sides. To me, one without the other isn’t as much fun. It’s really nice to be able to do both.”

On the real-life Claudia’s reaction to his song “Claudia”: “She’s more well-known than me, so it wasn’t like I put her in the spotlight. I did sort of ask, or at least I told her I’m going to call this song ‘Claudia,’ unless you’re not comfortable with that. She was cool with it. My favorite comment on that song is when people say, ‘This will be awkward when they break up.’ Thanks for the positivity.”

On how ready the siblings are for this tour: “We put a lot of hard work and thought into the live show. We really made the whole album to be played live. We knew we were going to be touring for this album a lot, so all of the songs were made with that in mind. Sometimes a song that sounds really good on headphones or in your car doesn’t always come off so well live, but this album isn’t like that.”

On “Ocean Eyes” becoming Billie’s calling card: “Now, whenever I’m writing songs, I’m analyzing them, thinking, ‘Is this going to be a song for Billie or for me?’ But that one was before I was writing for her like that. I started playing it with my high school band, and it just wasn’t working right.

“I think it’s a song that was meant to be sung by a girl. I just really wanted to hear the song in a girl’s vocal range, and hers in particular worked. She has such a beautiful voice, and sings in a way that you really believe her. As soon as she tried it out, I thought, ‘Oh, this is exactly how it was meant to be.’ But, of course, I had no idea it would catch on like it did.”

Chris Riemenschneider • 612-673-4658

@ChrisRstrib

Billie Eilish

With: Denzel Curry.

When: 7:30 p.m. Sat.

Where: The Armory, 600 S. 5th St., Mpls.

Tickets: Sold out.