The kinsman of a player-knight commits murder: he kills another knight by treason (not in a trial of arms). The player-knight knows the truth. Should his Honor take a hit if he denounces his kinsman?

I would say no, but his relations will certainly be hurt with the murderer (I could see a Hate (Whistleblower) directed trait), which would perhaps extend to the murderer's nuclear family (assuming he has one), depending on their family dynamic.

He might actually GAIN some minor Glory and an Honest check. Probably an Honor check as well.


T.

By the way, I'm using, perhaps erroneously, the word 'kinsman' to define a male relative of the Player-Knight, a direct cousin in this specific situation. I'm setting up the situation so that the PK will be forced to choose between kin and Honesty or Justice. Probably a conflict between Love (Family) and Honesty.

Aye, Love (Family) is definitely involved. I'd say it's a conflict between Honour and Love (Family), rather than Honesty. Killing a fellow knight be treachery is a crime against Honour, after all.

This is how I would handle it:
If asked: Did your cousin kill that man? This would be a Honest vs Love (Family) check. Denying the murder or that your cousin did it would mean you lose a point of honor. The winning trait would get a check.
If asked: Do you think your cousin has to be punished for the crime? This would be Just vs Love (Family). If Love wins you porbably reduce the sentence or may even deny that the cousin did it. The last would again reduce honor.

My two denarii:

Denouncing the kinsman would not go against Honor, but against Love (Family), as other people have already said.

I am not sure I would penalize Honor even if the PK would lie about it afterwards (unlike Cornelius). As long as he is not personally involved in the murder, he can look the other way, although his Honest might make him wish to come clean (as Cornelius said). There is nothing in Honor that says he cannot lie to a random knight, especially in defense of his kinsman, or simply say: "I have no inclination to speak with you on such gossip, Sir Knight." Now, if it is his own liege lord asking the question, then yes, his Honor and Loyalty/Homage (Lord) would take a hit if he lies. Note that the Orkney brothers can murder Lamorak, and everyone pretty much just shrugs and moves on, a blood feud is a blood feud. Gaheris manages to even claw his way back to court from murdering his own mother!

The Orkney situation, by the way, also implies that there is a tacit understanding that some murders are more justifiable than others. Murdering another knight simply because you want something that is his, why, that is robbery, plain and simple. While murdering him because he is the one who killed your father, why that is just avenging your father. Granted, you could have done it more honorably in a duel, but at least you had a valid excuse.