Yes, family history is considered IC knowledge.
The OP stated that he desired the name of the assassin to be added to the victim's family history, and the name of the victim to be added to the assassin's family history: "b) your character's death would be writen in your family history along with the killer's name/ also your character's name would writen in the family history of the killer"
There are many cases where the name of the assassin is never discovered, or the wrong person is blamed. How would anyone know who really did it? And if you were never identified, how would anyone know to blame you, so as to record it in your family history? Adding it to family history would be a horribly meta-gamey piece of information. It would be much better to simply tag the victim's history with "Slain by an unknown assassin".
If an assassination is successful, can the infiltrator still be caught? Kind of tangential (as it's a mechanical question), but could for instance Duke Kepler be stabbed by Sir Evil, who is caught in the act by, but escapes (or doesn't), the guards?
I do really like the idea of making character deaths more flavorful. "Character died" just seems really drab when it could be "Character executed by order of the Judge of Keplerstan". Speaking of which, how difficult would it be to add a check to see whether the cause (judge's name in case of execution, or character for hero's killed in battle) is known and include that in family history? Or is that basically the job of players and what the Wiki is there for?
