What is the legitimate legal ruling on retaliatory punishment (qisas) for a person who sexually assaulted my mother, given that he is influential and has illicit relationships?

This man is bad and aggressive. He should be advised and reminded of Allah. He can also be threatened with reporting him to the authorities to deter him. It is not permissible to assault him by beating or killing to avoid stirring up strife, especially since his family are oppressors. One can avoid associating with him or leave the village if possible, to guard against his evil.