Interfering in his married sister's affairs for advice and guidance

Q 3: Is it permissible for a brother to interfere in his married sister's life when he knows that her husband is the cause of her weak faith? (Part No. 25; Page No. 306

A: If the reality is as you have mentioned, it will be permissible to advise and remind them wisely and kindly of things that reform and enhance their religion and affairs, and rid them of doubt and suspicion. Allah (Exalted be He) says, Invite (mankind, O Muhammad صلى الله عليه و سلم) to the Way of your Lord (i.e. Islâm) with wisdom (i.e. with the Divine Revelation and the Qur’ân) and fair preaching, and argue with them in a way that is better. May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!