What is the ruling on a man thigh-intercoursing his twelve-year-old sister, and is this considered sodomy or fornication? And how can he repent from this when he is married and unable to [stop]?

Engaging in preliminaries to zina (illicit sexual intercourse) or sodomy is forbidden. Doing so with maharim (unmarriageable kin) is even more abhorrent, a greater transgression, and contrary to human nature (fitra). Immediate and sincere repentance (tawba nasuh) to Allah Almighty is obligatory. One must follow evil deeds with effacing good deeds and return to one's natural disposition (fitra). This act is a grave sin, even if it does not constitute zina or sodomy necessitating the prescribed punishment (hadd).