Legal Question in Criminal Law in Wisconsin

That sounds like a fictional story rather than a news report. Alternatively, you may be mistaken about what happened.

The boy would still be responsible for killing his sister unless the brainwashing made him unable to distinguish right from wrong, in which case the insanity defense would apply. That seems extremely unlikely. It is also possible for the prosecutor to drop charges in the interests of justice under appropriate circumstances, but he could not do so if the boy had already pled guilty as you say. (You also say that "detectives were about to convict him" after his guilty plea, but convictions are entered by courts and not by detectives.) I doubt that a prosecutor or a court would agree to dismiss charges in a case like this.