What is the religious ruling on a brother committing adultery with his sister in the absence of their parents, with his confession, in a country that does not apply Allah's Hudud (prescribed penalties) and Sharia?

Incest is one of the gravest sins and most heinous transgressions. The perpetrator must engage in sincere repentance. Concealment is preferable if his remorse becomes apparent; otherwise, his case should be referred to the ruler for punishment. The general public is not permitted to carry out the prescribed penalty on him.