His expression flinches, ever so slightly. To someone unaccustomed to Sans's permanent grin, it might be hard to catch. The subtleties are there, though. Something about that cut a little, but not in a way he's prepared to face head on. So instead, his aggressive front continues.

"I don't got a brother, pal," he says, like it's some kind of joke. "Don't you think I'd remember that?"

Except he's forgotten so much, already. But how could he have something like that and not remember it, in the rare moments coming back to him? How could something like that remain concealed? What about this laboratory makes him scared of the idea?