Grimes was friends with Shivon Zilis before she had twins with Elon Musk, according to a new book.
Grimes was "outraged" when she learned of Zilis' pregnancy at the same time as the public, the book said.
Grimes previously dated Musk and has three children with the Tesla CEO.
"Grimes considered her a friend and occasionally tried to set her up on dates," Isaacson wrote of the two women's relationship, adding that Zilis had invaded Musk's "tight-knit social circle," attended parties with the pair, and played video games with Musk.
The biographer said Musk had personally recruited Zilis to join his brain chip startup, Neuralink, in 2015 and she moved to Austin shortly after the billionaire moved in 2020.
Isaacson wrote that the Neuralink director decided to have children after Musk spoke to her about his population doomsday fears. Musk later offered to serve as Zilis' sperm donor, according to the book. In 2021, Zilis had twins with Musk via in vitro fertilization (IVF).
But, Grimes — whose legal name is Claire Boucher — only learned that Musk had fathered a set of twins with Zilis when it was first revealed to the public by Insider in 2022, according to Isaacson. The biographer said that Zilis also kept the knowledge of Musk's involvement from her coworkers at Neuralink.
Meanwhile, Musk and Grimes decided to have a second child via a surrogate even though they were going through a "rough patch," Isaacson wrote. At one point, Isaacson said both Grimes and Zilis were in the same hospital — Zilis with complications from her pregnancy and Grimes visiting the surrogate who was carrying her second child with Musk. But, neither women knew that the other was in the hospital or that the other was planning on having a child with Musk, according to Isaacson, who compared the situation to a "new-age French farce."
Later, when Grimes learned of Musk's children with Zilis, Isaacson said she was "outraged." The singer was in the process of having her third child with Musk at the time.
"They were all involved in contentious discussions over such issues as whether Zilis and her twins could spend time with Grimes's children X and Y," Isaacson wrote. "It was a mess."
The tension between the two women came under public scrutiny earlier this month when the singer accused Zilis of blocking her on social media.
"I have never been allowed to see a photo of these children until this moment, despite the situation utterly ripping my family apart," she wrote in the post on X that has since been deleted. Grimes wrote the post in response to a photo Isaacson posted on X of Zilis and Musk with their two children.
Grimes and Zilis later took to social media to say they were no longer feuding.
"Communication about the twins wasn't handled super well in the past, but I now totally understand what happened and totally forgive the situation," Grimes wrote on X on September 10, adding that she's excited for the two women's kids to "grow up together."
Zilis also thanked Grimes in a response on X for reaching out.
"So glad we had such a wonderful talk, was insanely overdue," Zilis wrote.
In Musk's biography, Isaacson said Zilis and Grimes play very different roles in the billionaire's life.
"Grimes is feisty in a fun but also fiery way, often getting into fights with Musk and sharing his attraction to tumult," Isaacson wrote, adding that in contrast Zilis had never gotten into a fight with Musk and spoke to him "in a low-key, intellectual way."
Musk and a representative for Grimes did not respond to a request for comment ahead of publication. Zilis did not respond to a previous request for comment from Insider.
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