Spoilers: Lelouch is a siscon as well as a lolicon.
Seriously, though, I guess Lelouch just wants to grant Nunnally-imouto's wish for a kind world because his mother is dead and dadoo pretty much tossed Imouto, Lelouch-mama and him aside, aka too much tragic stuff and less kindness. Since Imouto is his only sibling (in b4 Rolo), he wants to grant Imouto's only wish.
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Seriously, though, I guess Lelouch just wants to grant Nunnally-imouto's wish for a kind world because his mother is dead and dadoo pretty much tossed Imouto, Lelouch-mama and him aside, aka too much tragic stuff and less kindness. Since Imouto is his only sibling (in b4 Rolo), he wants to grant Imouto's only wish.