Kim just shared a funny video on social media showing her sister snapping her into a SKIMS bodysuit ... as Kim stands in a bathroom with her jeans down past her knees.

Khloe's tugging, yanking and pulling on the tail end of Kim's bodysuit ... doing everything she can to snap the darn thing together as Kim tries to find the best position to make Khloe's job easier.

Kim is recording their plight in the mirror as she says, "This is literally what sisters do" ... and Khloe sums it up best, "This is what girls do in the bathroom for so long." 😂

At one point, Kim seems like she's about to give up ... but Khloe gets one of the buttons to snap and they press forward ... determined to get Kim into her SKIMS.

Seems this whole ordeal is the aftermath of a potty break ... because halfway through, Kim wonders ... "What am I going to do if I have to pee again?"

All the hard work pays off in the end ... and Khloe gives Kim's butt a celebratory slap for good measure.