Umm we aren't sure if Kylie and Kendall Jenner got the memo that sister-on-sister fingering isn't so hot.

Yesterday, 17-year-old Kylie posted a very sexual Snapchat of her sticking her hands down her sister Kendall's pants. Yes -- you read that correctly -- sister-on-sister fingering.

We think they think they're goofing around -- but really they are just making everyone uncomfortable. Ahh sisters aren't supposed to do that!

In the short vid, the youngest Kardashian sister reaches around her big sister's waist and then heads down -- like all the way down.

Kylie is disgusting -- we already knew that -- but we thought Kendall was better than that. Guess not! We thought that Kendall might get out alive from the trashy Kardashys -- but it seems like she (and Kylie) are going down (pun intended) the well-oiled rabbit hole.

In addition to that fingering-banging vid, the gruesome twosome also posted a Snapchat of a full-on poolside kiss while hanging with (sort of brother-in-law) Scott Disick yesterday. At this rate, the gals will be releasing a sex tape, Scissor Sisters, by next week.

Most moms (if they saw their daughters basically fingering each other) would probably think they'd failed as mothers, however we have a feeling Kris Jenner is proud of how close her two youngest are. We bet Kris is on the phone with Steven Hirsch right now. Vivid Entertainment -- here they cum!

And that's not all folks ... Kylie posted a Snapchat of her new piercings, alluding to some new nipple piercings. Another classy move!