If he was a kid and the age gap was minimal, then I'd tend to agree. However, the MC is not a kid. It's not really said how old he is (or at least I missed it), but it was stated that the grandma was thought to be dead for 7 years at the time of the story, the parents even longer, since she took over from them. While I think said grandma is a horrible parental figure, I can't imagine she'd leave when all three were underaged. So yeah, he's pretty much assured to be in his mid 20s or older. To add to that, it doesn't look like they have another legal guardian (otherwise, how would he explain his finances? It would at least have to be mentioned), that means he is his sisters' guardian and has been for some time. So yeah, I'd absolutely have him take at least part of the blame.