He said to Sebastian "It's the first time we actually met" so he did not see him at that time, but probably got told about him (maybe Undertaker, dunno).
Yes, Lizzie knows. Obviously because she was in love with the real one and ourCiel just fooled her all this time, pretending to be the one she loved all these years before. What questions are there to ask, you mean like asking him why he did that or something? Honestly, she's probably not interested in hearing an answer from him lol. He blatantly deceived her.
So? Doesn't change that he did deceive her, and I doubt the main reason is something like "I wanted to protect you". If anything I'd say it had more to do with his insecurity about his own identity he wanted to get rid of. After all, he was his spare from the beginning, so he would've inherited the title even if he did not try to deceive everyone. All the while disregarding Lizzie's own feelings about all of this (even though he eventually did came to care a bit more about her). If anything, they probably did not spend much time together, he was sickly and apparently mostly stayed inside and she probably just played with her lover all the time instead.
He's also been telling so since the beginning of the series, he's very selfish. Also when he had that dream with his twin in the Green Witch Arc, he did clearly say this at the very end (though plenty of quotes could be taken from that whole thing):
"I am not so noble that I would stake my life for someone else, nor am I so forgiving that I would sit by and allow someone to trample me. I am a selfish and self-righteous human being! That's why! I to clear my own shame I used your power not for anyone else!! But for myself!"
Im a bit irritated ...
1. Those two kidnapped and then murdered, but the real ciel the one who died .. from the past memories chapter, the real ciel protected his twin, right?? And how can he know sebastian when he already died that time?
2. Lizzy , is she know that ciel has a twin?
They are known each other since childhood right? Why is she attacked ciel *us* just because she knew that he was not the real ciel,, ? Why not asking ,or searching some information ..?