My son is the one that was sexually abusing his sister and nieces

Do these children ever change?Do they ever even get what they've done is wrong? He doesn't want to talk to me because he knows that I see through him and his lies and manipulative behaviors.  He still speaks to his Father because he is just learning about the "true" character of this boy.  He seems to be able to "snow" his case worker right now too which is a big concern of mine.I don't understand why he behaves this way-does therapy work in these cases - I can't ever bring him back into our home because of his sister so what do you do in these cases where do you put them?  His sister is in therapy but the abuses that he inflicted on her are so devastating that we don't know how much we can help her.Has anyone else been through a case like this or know of any similar ones? Thanks