Curufin's shoulders are shaking with laughter as Kaiba teases him.

Don't think I'm not tempted!

On the other hand, he's an Elf, and Elves respect their elders. Not that Siebren probably is older than Curufin, but he has elder status, so far as Curufin is concerned.

Okay, we get the point, he says to Siebren. Use your own best judgement, and we'll trust you for it.