As Radar readers know, Bieber, 24, and much-older Kardashian dated from 2016 into the following year.
"Things between Justin and Kourtney were more serious than most people realized," the source insisted. "Their physical compatibility was off the charts, and they even started to bond about religion. Justin was hugely attracted to Kourtney, and she was really smitten by him. They had a lot of fun together."
The pair split, and Bieber went on to date Selena Gomez, 26, and then current fiancée Baldwin, 21.
But Baldwin isn't so secure with her quickie engagement, the source claimed.
"Hailey is very protective of her relationship and she has every reason to feel threatened," the source continued. "She's saying that Kourtney better stay away from Justin, and she won't hesitate to ban him from speaking to her altogether."
As Radar previously reported, Baldwin is also insecure about Bieber's longtime romance with ex Gomez.
"Hailey's dealing with daily onslaughts from trolls online, plus his own mom and others in Justin's circle who think Selena's his soul mate, not her," an insider told Radar.
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