What is the ruling on disavowing a sister due to her practicing prostitution and the mother covering for her, and what is the way to deal with this situation?

If what was mentioned about the sister committing indecencies is true, then she has committed a great evil and a clear sin. Adultery is a major sin among the gravest of sins. The questioner's jealousy over his sister is a sign of complete manliness and gallantry. What was mentioned about the mother protecting her daughter and justifying her going out is serious, and she should be advised wisely, and the gravity of the matter should be explained to her. The father, if alive, is her guardian and responsible for her, and he must be firm with her, for he is responsible for her before Allah, as Allah the Almighty says: "O you who have believed, protect yourselves and your families from a Fire" (Quran 66:6), and as the Prophet, peace and blessings be upon him, said: "Each of you is a shepherd and each of you is responsible for his flock." If the sister caused a separation between her parents, this is an act of disobedience added to her bad conduct, and all of this necessitates her sincere repentance.