Kaia Gerber is struggling to cope with the devastating loss of her brother and “best friend,” Presley Gerber, after he died in rehab Sunday.

“Kaia and Presley were insanely close,” a source exclusively told Page Six Tuesday, adding, “I don’t know how she’s going to go on without him.”

The 25-year-old model has been taking the loss “very, very hard” as she had an exceptionally tight bond with her older brother, the insider explained.

“They were way more than brother and sister,” the insider said. “She is heartbroken. They were like best friends.”

Kaia and Presley are the only children of supermodel Cindy Crawford and businessman Rande Gerber…

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