In the Name of Allah, the Beneficent, the Merciful.
May the peace, mercy and blessing of Allah be upon you.
I am a young woman who wears niqab, Allah be praised, and I don`t know if it is permissible to attend my brother`s wedding in a mixed reception hall that may involve music. The gathering will be limited to relatives because the bride is my cousin. My brother might get very angry if I don`t attend and tell me that there is nothing bad there; you can just come and sit there the way you are dressed and there wouldn`t be any problem. I tried to convince him to have an Islamic wedding, but if it won`t be, can I go at least half an hour for example and leave, or should I attend because this is a matter of obeying my parents (birr ul-walidain) who might get angry too…Please grant me the benefit of your advice, and may Allah reward you well. I would appreciate if the answer included evidence that mixed weddings are haram so that I can show it to those members of my family who believe that there is nothing bad at weddings as long as there is no dancing and it`s just family and relatives…Also, if it is permissible to attend my brother`s wedding, would it also be permissible to attend my cousin`s wedding? May Allah reward you well and increase you in knowledge and benefit…May Allah guide you to what He loves and that which pleases Him; and may the peace, mercy and blessing of Allah be upon you.

All praise is for Allah; to proceed:

Men and women gathering together in one place is not haram as long as there is no seclusion between one man and one foreign woman (for whom he is not mahram), without a third party in a closed room, and as long as the gathering of men and women in one place does not lead to anything haram like the improper dressing or tempting speech of the women, or the like of what makes people fall into temptation.

On that basis, as long as the aforementioned wedding or any other gathering involves none of these objectionable acts, there is nothing in Islam to prevent you from attending, even if it is mixed.

Likewise, if you found some objectionable practices and you attend with the intention of advising them and calling them to proper Islamic conduct, then actions are by their intentions.

We advise the sister in question to treat her relatives with the utmost kindness and to join the bonds of kinship. Indeed, the greatest of acts in this respect is to participate in their weddings, to encourage them to be committed to the religion and to proper dress by setting a good example, and to show that her hijab does not prevent her from maintaining good relations with her family and treating them well.

And all success is from Allah.