Well... Uh...

Basically, an orphaned child is in search of his real family.
He meets a girl, who claims to be his sister from a distant planet.

She tells a flashback:
Their mother died during a battle of the planets, and only their father was left.

Unfortunately, although the father won the battle, he was infected with a virus, which mutated him horribly over 20 years (age of main characters). He had no choice but to exile his two children to different planets; one of which was Earth. :)

That's about all I could come up with at the time