I'm not sure, but could you be
a TRUEL, or duel between three people, as in the famous puzzle.
I am an event,
relations I set.
Make most to fret,
threat because they get.
A duel/truel is (was) an event due to some bad relations between two/three people. In olden times, a threat might easily result in such an event.
I sound like evil,
but actually I'm not.
Maybe as a concept,
but it is not my fault.
The word sounds like CRUEL, which is evil. The concept of fighting a duel/truel is a bad thing, but it's the fault of the people doing it (or the society encouraging it) rather than the duel/truel itself.
I sound like fact,
reality, exact.
Nothing do I have
to do with all that.
The word truel also sounds like TRUE, but they're completely unrelated in meaning.
Honor and villainy,
beauty' n' unsightly,
a great example of me
contains of these the three.
This is a reference to one of the most famous truels in cinema, The Good, the Bad, and the Ugly.
My brother who has two,
is way more known, too.
Through me he who goes through,
does not need know kung fu.
Duels with two people are better known than truels with three. Maybe, given Hint 1, this might be something about boxing or martial arts, which could be seen as a sort of (usually friendly) 'duel'.