Kylie Jenner let her fingers do some walking down her sister Kendall Jenner's shorts, and then -- because she's a giver -- posted it on social media for the world to see.
The Snapchat looks like it's behind the scenes of a photo shoot ... and shows the sisters doing a lot of sexually charged goofing around -- and then it gets all touchy feely.
For some reason 17-year-old Kylie reaches around her big sister's waist and then heads south. Way south. Kendall let's out a shocked, "Kylie, ohhh?!"
As Principal Rooney put it ... "So, that's how it is in their family."
[H/T: TMZ.com]
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