1. That was rather overkill, burning the place down and leaving an assassin with a sword and an endless supply of throwing knives, willing to fight in a burning and collapsing building, behind. (While MC's sister was painted as suspicious before, there's no reason to hide her if she was the mastermind, so I say she's a red herring; I think I'd appreciate if the artists hid the real culprit in the background in chapter 3, maybe I'll check; p.s. doesn't look to be the case, with the exception of one panel all guests shown are male, that's a disappointment)
2. Whoever killed them had no way of knowing they'd come there, yet before setting the place on fire managed to seal all the windows with boards, broke down the stairs and evacuated all the guests in a matter of minutes. And all that silently enough that they wouldn't notice.