But I do understand that someone who is that evil simply cannot be rehabilitated, and she presents a clear and present risk not only to society, but to the human race. Plea bargain notwithstanding, the authorities should be monitoring her every single move, and the moment she does anything even remotely illegal (even just jaywalking), that's it. You send her to the nuthouse for life.
And don't argue that she was battered or dominated by her husband into cooperating with him. That's nonsense. If your husband turns to you and says "hey, you know what would be great? If we raped and murdered your sister," you reply to him "that's cool, I'm just going to go to the toilet now," and then you call the police. There is no level of duress that could make someone actively participate in the rape and murder of her own sister.
So that's it. Someone who has done what she has done is simply not a human being, at least not as we understand human beings. She should not be allowed anywhere near any person under the age of 35 for the rest of her life, period.