Is it obligatory to treat one's parents harshly and rudely in speech to deter them from evil, and what is the ruling on intimidating them to achieve that? And does a person incur sin if an evil takes place in their home without their knowledge?

This dangerous reality requires you to be patient and wise in dealing with your family and to pray for their guidance. Be sure to advise gently and kindly, and do not resort to exposing them. However, there is no harm in threatening if it is to deter evil. If it is impossible to reform their situation and you find a way to live alone, then do so while maintaining filial piety and kinship ties.