In the fiction, it is either true or false that the brother was evil. That truth doesn't change.
If it was
actually true that the brother was evil, or that the brother was not evil, then all evidence would always be consistent with that truth (even if it may sometimes be misleading). That's part of the definition of what makes something
true, is that it is the way it is regardless of who is aware of it.
Before any investigation toward discovering the truth, that truth must already be defined in an absolute sense. How someone goes about
discovering that truth cannot possibly have an effect on the nature of the truth itself. The contents of a box must already be defined
before you open it and learn the contents, if you're living in any sort of normal linear-time objective reality.
At the table, that element of the shared fiction has to be settled through some process or other - fictions don't just write themselves! At my table, it was settled by narrating the consequences of the failure. Up to that point, the relevant element of the ficton hadn't been written, and so - not having been written - I was in no better position to know it than anyone else.
At your table, the
true nature of the universe in which the characters exist is that there
is no objective reality, and anything a character does can have spectacular repercussions at any point along the time-stream. It is literally,
objectively true that the brother retroactively
became evil-all-along as the result of a failed perception-based check.
We know this because we have access to the source code by which that reality was generated. The
characters are entirely oblivious to the fact that they live in such a world, because malevolent higher-dimensional entities are conspiring to trick them. They live in the Matrix, or in the philosophy of Descartes.
And all that is fine, whatever, you can have fun however you want. It does necessarily mean that everything in that game is determined by fiat, though. There is no DM who can make a judgment call about what makes sense based on everything they know about the world, and assign a probability if necessary, in order to figure out what
should happen as the result of any given action. Your game lacks the pre-requisite of a DM who actually
does know all of the relevant factors involved in making that sort of call.