Somehow it's remind me when kids bully their classmate. And when the victim almost died, or live something dangerous (suicide, accidents,...) they start to regret it. I think that how it is for the father and brothers.
As for the brothers and the way they acted. While I DON'T accept the abuse, I understand their feeling a bit. I think that for them Penelope were just an outsider who were trying to steal their sister's place. I think that why they hated her so much. I also hated my stepmother when my dad remarried after divorcing with my mother. I said a lot of mean things to her when I was young. Since for me she was the reason of their split and she was trying to take away my mom place.
That why I understand how they must have felt quite well.