YES! ANOTHER SIBLING THEORY! These are my favorite.
Yes, it is highly probably that is related to another Shard in some way, shape, or form, and probably Odium, but really, we don't know; it might be a mix of Honor and Cultivation.
This ^ seems to suggest the third Bondsmith would be some form of Odium connection, but there's nothing hard and fast as far as I'm aware.
We will find out soon, though: