For some reason i really want this manga to subvert expectations and make it turn out to be her mother that DID create the fire. Wonder how guilty the chick would feel
A completely wild guess from me, but in your hypothetical subversion, I think the author will go one step further:
FMC's mother did start the fire, but was already psychologically stressed out and goaded on by Mitarai, probably by planting doubts about her husband's (in)fidelity and/or gaslighting the mother.
@Sonaldo I thought it's because he didn't want her bandaid to get wet and maybe cause an infection. He does seem to be traumatized by something though. Seems like the dad might have beat him or he was bullied