Billie Eilish took to her social media early on Monday to celebrate the birthday of musician, longtime collaborator, best friend, and brother Finneas O'Connell.
The singer-songwriter and record producer turned 26 on July 30, and his sister, 21, shared a loving tribute to him on her Instagram.
She posted a throwback that took it way back, all the way to her birth, when Finneas was a little four-year-old toddler and adoringly gazed at his baby sister.
While she was dressed in a onesie, little Finneas wore a tie dye shirt and sported dirty blonde locks, laying beside his sister while smiling at her.
"My best friend forever," Billie wrote. "Will never love anyone as much as I love you @finneas Happy birthday you are the best thing I have."
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Fans loved the heartfelt message for her brother, but were even more stunned when they realized the toddler in the photo was Finneas and not Billie herself and remarked on the resemblance.
"Why, I thought Finneas was Billie? Happy birthday Finneas," one commented, with another also saying: "I thought Billie was the one in yellow and was so confused why she was older than Finneas in this photo," and a third joking: "My Brain wasn't Braining for a while."
Several left sweet wishes for the Grammy-winning hitmaker, leaving comments like: "Literally best friends forever. The duo that keeps on giving us magical and beautiful music," and: "A Leo, we love a Leo! Happy birthday finneas!!" as well as: "So glad that you have him and he has you."
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While Finneas has been an actor and musician long before his work with his sister began, his career took off when he gained attention for his collaborations with Billie, from the initial success of her 2017 EP Don't Smile at Me, all the way to their latest hit, 'What Was I Made For?' from the Barbie soundtrack.
Speaking with Billboard in 2019, days before Billie released her debut album When We All Fall Asleep, Where Do We Go?, which led to their domination at the Grammys, he spoke of their relationship as a producer and artist.
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"Being able to hear an artist and emulate them has been a huge part of being successful as a producer and co-writer," he said. "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."
In a 2017 interview with Harper's Bazaar, Billie elaborated on their songwriting process, stating: "My brother and I always write together pretty much, and we write so much that it's never like we don't have new songs."
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She attributed their relationship with music to their mother, actress, teacher, and screenwriter Maggie Baird. "My brother also started writing at the same age I did.
"I feel like I always wanted to write songs just because my brother did, and also my mom did. She taught both of us how to write and so I always have high standards for writing. If I didn't, I'd have 2,000 songs."