You heard it hear first. Ashley's soul gets removed from her body in 4A, and then Andrew destroys the evidence by chopping up her body for lunch. Since the demons can return souls to bodies, it makes sense that Ashley's tar soul could be inserted into another human's body: somebody who is not Andrew's sister. That way Andrew can non-incestually fuck his sister, and we get the happy marriage ending with 5+ non-inbred offspring.