What is the ruling on attending my brother’s wedding, where there will be a female band playing music, knowing that my brother’s fiancée and her family do not see this as forbidden, and how can this situation be dealt with wisely?

Permissible singing at a wedding is that which is not accompanied by music, and it is permissible to beat the duff (tambourine) during it. As for singing with musical instruments (ma'azif), it is not permissible, even if it is by women.

It is advised to try to convince the girl's father, in a kind manner, not to allow this objectionable act (munkar). If he is not convinced, then obedience to him is not permissible, and it is not permissible to attend the place where the munkar is occurring. For whoever can be in a part of the house where they neither see nor hear the munkar, there is no blame upon them.

There is no obedience to a creature in disobedience to Allah, and it is not permissible to please people by incurring the wrath of Allah. Whoever seeks Allah's pleasure, even if it displeases people, Allah will spare him the trouble of people. And whoever seeks people's pleasure, even if it displeases Allah, Allah will entrust him to people.