All that stuff around sin feel uncomplete, like, they were trying to add the old DS 1 system but never ended it. But, looking upon lore, it makes sense.
Darkmoon blades, the "judges", are only pale copy of what they where with the fall of Gwendolyn, and more generally, of Gods: No Gods to pray, no sinner to hunt (How can you sin if there is no trust to believe in?).
And that's the same for Velka. The Gods started the first sin with the linking, but now they are gone, she has nothing more to do here.
The fire fade, the Age of men is coming. What once was a sin is now only a fable, and as the darkmoon blades fall, the blue sentinels rise as their "Human" counter part.
I would have loved to know more about Velka and Carim. This place looks like interesting. But I suppose we never will.