I keep thinking of Tywin as eldest, but I can't actually remember if Genna talks about his as a younger or older brother... He certainly *acts* eldest, but that could be just the force of his personality. (I should know this, dammit! What kind of Twyin fangirl am I, really?)

I do think Cersei has a bit of Joff's sadistic streak, too -- the scene with baby!Tyrion, for instance. Granted, she had just lost her mother and had her father lock himself away to mourn, so it's not like she was torturing Tyrion for *fun*, but still, that aspect of her personality always made me profoundly uneasy. I suspect she could've turned out as bad as Joff if she'd been allowed free rein (or reign, heh) and had her psychosis mistaken for strength and encouraged, the way Cersei seemed to be doing it with Joff.