The only issue is whats the point.
No matter how you slice it, keeping her locked up is nothing but revenge.
we have said it is morally wrong to act on revenge or use it as a basis in our legal system.
Im not saying we shouldn't have revenge based punishments. Im saying if we are going to punish other people for acting on revenge (like beating within an inch of their life someone who killed their child), then we cannot have our legal system act as an agent of revenge.
If we are going to say revenge is acceptable and a valid emotion the state sponsors, then fine, lets stop arresting people when they succumb to that emotion.
If the victims sister had killed this woman she would be in Jail, why? If we claim its ok to make the woman suffer as form of revenge, why would we then arrest someone (and make them suffer) for doing the same thing?