[Guests should greet the host - that's just proper manners. The thing is, Sebastian very much prefers angels and demon scum (and Lucifer qualifies for both) far, far away from his master. Sebastian will do the host greeting for the two of them.
So after serving Ciel something to eat and drink and seeing him off to do some forced social mingling, Sebastian approaches the piano.]
What a lovely set-up, sir. [Yes, we're back to "sir". What are you gonna do about it.]