Kylie Jenner -- Gives Kendall Jenner the Ole Reach Around (VIDEO)

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.