I know it's forbidden to stay angry with another Muslim for more than three days except for a valid reason, so can I stay mad at my sister if she talked about me to her friends behind my back?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Your sister, whom you mentioned in the question, needs good advice and does not need to be deserted. However, if she does not listen to your advice and insists on committing sins, then you may desert her even for more than three days.
Ibn Taymiyyah said: "Whoever commits major sins openly deserves the punishment of being deserted or any other punishment if there is a preponderant benefit in deserting him, so he should be deserted for this religious benefit to be achieved as much as possible.
Ibn Taymiyyah means that abandoning a disobedient person is not the main purpose but the main purpose is to correct him and to repel his harm from the people. If achieving this purpose is possible by advising him or by rebuking him, then he should not be deserted; otherwise we may desert him for even more than three days.
Ibn Muflih wrote in his book Al-Adaab Ash-Shar'iyah: "Imaam Ahmad said from the narration of Hanbal: "If one knows that someone commits a disobedience while knowing that it is a disobedience, then a peson is not sinful if he is harsh with him, otherwise how can this disobedient person know that he is wrong if he does not see any friend who denies that from him or who is not harsh with him."
Besides, Imaam Al-Qurtubi said: "Abandoning one for his acts of disobedience and innovation is compulsory until he repents from that. There is no disagreement among the scholars on this issue."
Allaah Knows best.
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