Which one of his siblings is most like Lelouch?
Some possible candidates for me:
Is it Clovis, we really didn't get to know him too well, but we know they played chess a lot together of kids, Lelouch always beat him, he knew how to put on a production like Lelouch did as Zero. He has a ego of wanting to be showmen like Zero. But Lelouch took him so easily, so he is not that smart, Lelouch would never get taken down so easily.
Schneizel, he is one who best could keep up with Lelouch in plotting and planning. They both thought Charles was a mad man, and needed to be taken down. While he was legitmately angry and upset of what happened to Clovis (if not, he is a great actor), he is really selfish and only looks out for himself. While Lelouch can be a little selfish, deep down there are people he cares about, especially Nunnally.
Cornelia, if you go by the you are only as strong as your weakness theory, they both can be very cold and ruthless, excellent generals, but also have soft valunable sides, with the same weakness, their younger sisters. For her it is Euphie, for Lelouch it is Nunnally. They both had about the same amount of good and evil, but as the series progressed Cornelia became less ruthless, while Lelouch became even more ruthless.