Code Geass : A News and Fan Community

In some of the supplemental material CC spoke as if she was around in the 1700s when the American Revolution (called Washington's Rebellion in Geass-world) was going on, so she's at least that old. The most likely speculation as to who she is that I've heard is that she's Joan of Arc. That kind of rules out them being siblings unless Marianne was also immortal and either faked her death or lost her immortality (both are unlikely.)