What is the ruling regarding the sin of a person who knows the impermissibility of his fiancée speaking on the phone and sitting alone with her fiancé, and has advised his family and reminded them of the hadiths and proofs, yet they insist on it? Is he sinful if this act is repeated in the house in his presence, and what should he do?

The fiancé being alone with his fiancée before the marriage contract is a forbidden and reprehensible act. If you have explained this to your family and they have not responded, then speak to someone who can prevent the reprehensible act, such as the father. If that does not help, then prevent the reprehensible act to the best of your ability by not allowing the fiancé to be alone with your sister, and by sitting with your sister when her fiancé is present. Mere disapproval is not enough, for the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart — and that is the weakest of faith." If you are unable to prevent the reprehensible act, or if preventing it would lead to a greater harm, then you are excused.