As far as Islamic Shari`ah is concerned, a Muslim is allowed to attend the wedding of his/her non-Muslim relative, held in a church so long as this attendance does not involve participating in any haram (prohibited) action.
Sheikh Ibrahim Salih Al-Husaini, Mufti of Nigeria, states: Almighty Allah says in the Qur’an: (Allah forbiddeth you not those who warred not against you on account of religion and drove you not out from your homes, that ye should show them kindness and deal justly with them. Lo ! Allah loveth the just dealers.) (Al-Mumtahanah: 8)
This verse, and many others in the Qur’an, lay down the main basics in dealing with non-Muslims. Tolerance and respect are among the main principles that govern the relation of Muslims with the People of the Book (Jews and Christians). It goes without saying that the People of the Book are permitted to attend Muslims’ weddings that are held in mosques in case there is a relation of kinship between them and the Muslim bride or groom, such as the case when the bride or the groom has a non-Muslim father or relative.
During the time of the Prophet (peace and blessings be upon him) the Christians of Najran came to the Prophet as envoys, the Prophet (peace and blessings be upon him) welcomed them in his mosque and they stayed there for a while.
Based on this, we can say that a Muslim is allowed to attend the wedding of his/her non-Muslim relative, held in a church so long as this attendance does not involve participating in any haram action. This means that a Muslim is not allowed to repeat the hymns said by the priest or other non-Muslims, for that goes against the main precepts of Islam. It is to be stressed also that a Muslim woman is to abide by the code of dress established for a Muslim woman. She is to cover all her body except her face and hands. Moreover, a Muslim should not eat or drink anything haram and should avoid any mixed gatherings that place his sense of religious commitment at stake.