"She's been texting him sexy selfies and flirting non-stop when Khloe's back is turned," an insider exclusively told RadarOnline.com.
"Kylie gets a huge kick out of it and can't help herself."
This isn't the first time Jenner, 19, has gone after one of her sister's love interests. The insider points out that in the past she's managed to grow cozy with Kendall's guys, Kourtney's ex, Scott Disick, and Kim's husband, Kanye West.
However, this time, Jenner's infatuation with 26-year-old Cleveland Cavaliers star Thompson seems to be different.
"She's got it bad for Tristan and has even told friends that he's way too hot for Khloe," said the insider.
"If he made a move on her she'd totally go there," warned the source, adding the situation could become explosive quite quickly, especially since Kardashian, 32, and Thompson's relationship has hit some major speed bumps.
"Tristan dumped his pregnant girlfriend for Khloe, so it's certainly in his nature to cheat."
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