What this is saying is that when you fully believe the truth, that you are not a victim, that forgiveness has been performed, and that atonement has been claimed, there MUST be a DEMONSTRATION OF HEALING, in both yourself and your brother, to SHOW that what seemed to happen in sin has had NO REAL EFFECT.
This correlates with a part in the bible which says that signs must follow those who believe, meaning, supernatural manifestations of truth must be exhibited as proof that what you believe is true. Not just a belief intellectually, but a DEMONSTRATION.
"You must attest his sins have no effect on you to demonstrate they are not real."
If your brother does something to you and it has an effect on you, this proves his guilt. Jesus is asking us to prove his innocence, by not allowing such effects to be shown.
And just think about this:
"Forgiveness is not real unless it brings a healing to your brother and yourself."