Well, I think we've determined that the real question that needs to be answered is how much you are aware of an attack through the Uncanny Dodge ability.
Looking at the uncanny dodge description it seems pretty clear that it lets you react to an attack before you can sense it:
... can react to danger before her senses would normally allow her to do so.
It's reasonable to assume that Uncanny Dodge let's you know that you are being attacked (Otherwise, how would you know to react?), but nowhere in the feat does it say that you get to know where the attack is coming from, what form of attack it is, or who is attacking you. The whole point of the ability is that you can
uncannily react to an attack with less information that most people need to defend themselves.
Consider this analogy:
Instead of being shot by an arrow from an invisible archer, assume, instead, that the target is about to be attacked by someone with improved invisibility. The target knows the attack is coming and readies an action to "disarm anyone who attacks me". If the target fails listen and spot checks, etc. to discern the attacker, she will not know what direction the attack is coming from, what kind of attack it is (Sword?, quarterstaff? trip? Grapple?), what kind of creature is attacking. (Human? Pixie? Demon? Stirge?), or where the attack is going to hit her, (Face?, Torso? Feet?) She can't know all of these important details without being able to
see the attack (not just know that it is happening).
Under these circumstances, would you allow her to attempt the disarm? I would not, even if the attacker were trying to stab her with a hand-held arrow and she had the deflect arrows feat.