we see that the dead parents, at least the mom, is kinda just the worst. She beats her son when he's young and manipulates him as well. His sister obviously misses their parents and it kinda reinforces this idea in the brothers head that everyone, including his family (even his sister), is out to get him/against him. Obviously his sister never abused him but she's seen as "an outsider" to his personal struggles. I can't read JP so some of this is just interpreting the art.