there's 2 chance right
1. it's the brother that are the asailnt
2. it's asailant that disguised himself as the brother
but the 2nd seemsly have higher chance because there's no way the brother suddenly speaking just because of it
the premise sounds stupid, dunno why the priest say copy is bad skill. just let people train to copy some op skill everyday. there's no difference with letting op skill user train everyday.