[Irenesharda]

Here's a more realistic twist: what if the Emperor was behind the whole assassination in the first place? Think of it. What if he too wants to conquer Earth. He would need a cause to go to war and a bunch of supposedly "rebellious" knights to carry it out. So, he secretly gives orders to Saazbaum, one of his trusted knights, to have some Martians spies stage an assassination. His granddaughter and heir is killed, but Mars regains their "birthright" and conquer the Earth. The same Earth that took his son away from him.
As for who would be his next heir then? He can simply delegate the throne to the knight that has gained his favor. Asseylum is too soft to become Empress anyway. She's better served as being a martyr for her people.

Hmm, it's actually a pretty sound theory.