Is it permissible to attend the wedding of a brother who is marrying a girl who is neither Muslim nor a Kitābī (People of the Book), knowing that abstaining from attending might cause a significant rift?

It is not permissible, according to Islamic law, for a Muslim to live with a non-Kitābī (non-People of the Book) disbelieving woman, nor to marry her. Marriage to such a woman is invalid by consensus (ijma‘), and cohabiting with her is a form of adultery. The man must be advised to sever his relationship with her, and attending such an invalid marriage is prohibited, based on the Almighty’s saying: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them. Indeed, Allah will gather the hypocrites and disbelievers in Hell all together." Muslims should be reminded of the importance of submission to Allah and obedience to Him, and alerted to the danger of abandoning prayer and drinking alcohol. Moreover, residing in non-Muslim countries entails risks to one’s religion and worldly affairs.