Making some sealed envelope predictions as I enter the final act (two achievements worth of game remain):
Spoilers, naturally
I don't think Catherine was supposed to have died at all. I think it Katherine was the target. The murderer didn't know there were two of them. Which means that the murderer wasn't from the Abbey in the first place. Am I making this my favored theory because I don't want any of the sisters to be the culprit? Maybe. But I think it must have something to do with the circumstances around Katherine's husband's death.
The details are the same. Or... at least I think they are. Beheaded. The murky 'politics' surrounding whatever's happening at Linbarrow town. The conspirators at the beginning of the game (was one the future Richard III?). The way the sisters have emphasized that basically anyone could come and go as they please into the abbey. The odd way the town has turned against them. There are petty grievances going on within the abbey, but the way everything about the surrounding circumstances outside the abbey seems far more suspicious.
Well, Hedwig's just found Flora, and I think I'm just about to enter the endgame.