Consensus opinion is that your bound sin isn't supernatural for the purposes of harming sins.
The description implies that you could theoretically bind more sins during the game but doesn't mention how. Sounds like a group-by-group question.
Passive
You have the forbidden ability to bind Sins. You have the obedient essence of a minor sin bound to you, under your control.
Your Bound Sin is animalistic in form and ability - you can determine what form it takes.
It can understand language but cannot speak, and is invisible to humans. It can follow you around at short distance, follow simple orders, and uses your action ratings to do anything. Its general capabilities are CAT 0.
If it takes any stress, it is banished for the remainder of the scene, however you can psychically absorb all stress taken for it instead to prevent this effect.
In a conflict scene, you may sacrifice your ability to act on your turn to allow your sin to act instead, giving it commands. Otherwise it doesn’t act independently in these scenes.