like it's straight up hysterical, you've got this greasy, charming guy who's got everyone at the local bar wrapped around his finger that's shady and Lore Implied and may or may not be willing to kill a child, who sleeps on the floor and doesn't shower... and also he reads his brother bedtime stories. and helps his brother make costumes for costume parties. and records him pretending to kickflip so they can edit it to make it look real. and cuts his apples into slices for him. and gushes to anyone about how cool he is. and he's the only one "[papyrus] can be his true self with". like the contrast is so good, you can't just shrug those moments away.
papyrus calls him nii-chan! NII-CHAN!!!!!! he tries to think of what it would be like for sans to be his mother and his conclusion is "doing the same things he does now, but without me having to ask." like come ON.
because that dynamic also makes it all the more sweeter whenever papyrus takes care of him right back. when he cleans the house and pushes him to get a job and take care of himself too, and yes, cool chill older bro who looks after him, but when he finds him crashing on the couch at a party, he still tucks him under a blanket. they fucking love each other so much and i am going to explode. apple slices.. GOD.