Kylie Jenner let her fingers do some walking down her sister Kendall Jenner's Pants

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]