Yes, my lord, [he obediently agrees to the order. His smile widens at the question (it's wise to ask it; Ciel should have thought to do so a long time ago) but that's the only delay to his answer. Without even once averting his eyes from Ciel's:]
No.
[Sebastian keeps many things from his master. His thoughts, information about himself, things he's done or encountered while in Ciel's service... More so back in London than here, admittedly, because life here moves at a slower pace, with fewer secrets to keep. Normally, "no" would have been the end of it; no more, no less information than Ciel demanded. But right now... ah, his little master, please don't distrust him so.]
I never met your parents. I was neither involved in any of their dealings nor instrumental in their passing. [He never said he was, did he? Only implied and let Ciel draw his own conclusions.]