Initially, Sans is just prepared to tell him to go, and to end the whole thing right there. He's clearly having some kind of mental breakdown, here, and there's no need for the warlock to be there and watch the whole thing. Yet, that last comment, is what causes him to tune into things again, at least enough to ask a question.

How is it that everyone he meets seem to be having the same disconnect from reality at the same time? It's exhausting.

He actually forces him to meet Papyrus's look, and all traces of his previous fury are gone, for the moment. Instead he just looks really tired and really done with existing.

"I don't... get what you mean by that," he says. "The brother part. Could you do me a favour and explain it?"