Is it obligatory for the questioner to boycott her brother again because she suspects his wife has reverted to Islam, especially after her brother claimed she had, yet she saw her dressed immodestly?

It is impermissible for a Muslim to shun his brother for more than three nights, except for a legitimate religious reason, based on the Almighty’s saying: "Indeed, the believers are brothers." (Quran 49:10). There are also Hadiths that prohibit shunning and severance of ties, among them the Prophet’s (peace be upon him) sayings: "It is not permissible for a Muslim to shun his brother for more than three nights," and "Whoever shuns his brother for a year is like one who has shed his blood."

However, shunning is permissible if its cause is a religious matter, intended for deterrence and return to the truth, as occurred with the three who were left behind. Imam Al-Bukhari titled a chapter in his Sahih (definition): "Chapter: What is permissible regarding shunning of one who disobeys."

Accordingly, there is no harm in shunning and boycotting one's brother until he refrains from holding onto the bond of the apostate disbelieving woman, based on the Almighty’s saying: "and do not hold on to the bonds of disbelieving women" (Quran 60:10), provided that the shunning is for the sake of Allah and not out of personal desire, and that it is confirmed the wife remains in her disbelief. It is worth noting that it is impermissible to have intercourse with an apostate wife, and the marriage is annulled upon the expiry of her waiting period if she does not repent before then.