@ nsl41288: The nun is dead. She wanted to die. In order for the nun to be a possibility, CC would have had to give her a Geass right after obtaining the Code. However, you bring up an interesting theory that Marianne may be older than she appears.
@ Naekino: What? CC was the one that Marianne formed a contract with.
The thing I find strange is ...
... Marianne doesn't seem to be as 'motherly' as Lelouch remembers. She doesn't seem to particularly care what's happening with Lelouch. We don't know if she is on Charles' side, Schneizel's side, or if she is independent.