shocking thing is, sister wasnt sorry about what happned..

I'll try to keep this short..... My best educated guess about her attitude towards sonny is this.... They both were close in age, maturing mentally and physically from teens, to young adults, living in a house with an dominating abusive father, and a weak and unstable mother. At some point I imagine, both siblings began to protect each other (as shown during the mirror scene when the father begins yelling at sonny and patrica gets up to block the father, protecting sonny.) Before sonny was possessed, I would imagine there was a closeness and sense of protection installed in both of them from the many years of mental and physical abuse they suffered. They probably covered for each other often or maybe took the blame or beatings for each other during troubled times. sonny was just starting to gain his freedom and probably dated a girl or two, where as, patrica still had some years to go before she could get away from dear daddy-poo. When sonny became possessed, the demon made sexual advances to be disgusting and sinful. The sister however, in a confused state (and probably abit messed up from the house's evil) accepted his advances. I am pretty sure she was a virgin before sonny, in fact, her father probably never allowed her to even date guys, perhaps the father at some point made sexual advances at her, so maybe she did not think it wrong to love someone that way, even if they were related. Anyways, just my opinion and/or theory, hope it offers some sorta insight.