Nice puzzle! I solved it quite smoothly without having to make random guesses (although assumptions were inevitable).
Nitpicks (or wishes):
- Personally, I wished that some of the victims would need to be identified by their behaviors / locations (besides the twins) / belongings.
- It'll be more realistic and interesting if more than one victims remained conscious longer and try to save themselves / protect significant others - like, large body mass is correlated with higher tolerance; and this could possibly leave more info than just forensics evidence.
- I thought the location/identity of the accelerant would be used to solve some question.