That is a pretty intriguing title, I have to say.
And seriously...That backstory...It has just SO MUCH POTENTIAL, that I wish I can see more of this. I definitely hope there's a sequel in the works.
Sure, why not?
Zero Degree: parent and child
First Degree: blood-related siblings with the same biological mother and father
Second Degree: Half-siblings, sharing at least one biological parent
Third Degree: Cousins, which is actually acceptable in some cultures
And we already have a term for "adopted, not blood-related siblings"--inseki