Here is what I think. I agree with most theories, however if we are going with the timetravel thing, then perhaps Isis and Osiris were not on Danu Talis at the fall, and afterwards they managed to acheive a state of existance outside of the time stream, like Marethyu does. After this, they attempt to raise Sophie and Josh for their scheme, and return to Danu Talis to do this. Then, the advanced outside of time stream isis and Osiris are killed by Josh Newman, and by combining the swords, he becomes Marethyu, one of the most powerful beings in the universe, literally one with the magic of Aether, or time. If you follow this theory, the pre-outside-of-time-stream isis and osiris are still alive. by furthering this theory, this Isis and Osiris are living in hiding, and are not known by the Elders. It is only later, after they achieve existance outside of the time stream, that they move part of their existance to Danu Talis, where they build up a reputation as Aten and Anubis's uncle, one which they had not had in their original forms, when they were stuck to the timestream.