It's like someone's taken all the air out of a very angry balloon. All the wind out of some hate-filled sails. With Sans apologizing, the cycle is broken. Mettaton isn't his cousin; he can't hold a grudge when the other party doesn't give him a reason to.

Sans is tired. Mettaton is tired. Exhausted, really. He can hear Sans' burnout just as clearly as he feels his own and all he can do is limply accept the fingers poking through his hair.

He'd argue about the things skeleton hands are and are not great for, but he doesn't want to think about how he knows what he does. That's done. It's over.

...it's all over.

Mettaton's quiet for a long while as Sans works. He's online, obviously. There's still a few tears here and there, but they've mostly flowed to a stop. How dramatic of him. He's clearly just thinking. Reflecting. Letting his own relief and exhaustion settle into his metal bones.

Finally:

"'m sorry." It's a weak crackle out of his speakers, but what with how quiet they're both being, it's easily heard.