Self-deprecation or not, Sans actually does find it in himself to laugh. There's nothing really funny about sarcasm, not when you get down to its roots, but commiseration is always a good reason to laugh.

"I tried that once. Protip: having a brother kinda throws a wrench in things." Not that Sans sounds even remotely unhappy about that fact. A little wistful, perhaps, but not unhappy. "Do you have any family, Ry?"