@EmzyOfNeverland Right, I feel like a lot of the morality cooks down to "Don't because Grimm". Thanks for addressing that.
<Snipped quote by tobiax>Old English synn (noun), syngian (verb); probably related to Latin sons, sont- ‘guilty’.
You don't necessarily need sin to be a transgression against a possibly existing divine law (which, if the two brother Gods who created the world exist, likely does). Besides, it's not like the Christian conception of sin is one-to-one with other religious conceptions of it anyway.