Is it permissible for me not to fulfill my pledge before Allah to abstain from attending wedding halls, given my husband's request for me to attend his brother's wedding?

A person's statement, "I pledge to God," is both a vow (nadhr) and an oath (yamin), and it must be fulfilled. If the pledge is to abandon something permissible, then violating it is permissible, but it necessitates expiation (kaffarah). However, if the pledge is to abandon something forbidden, then it must be fulfilled for two reasons: the inherent prohibition of the act and the obligation to fulfill the pledge.

Wedding halls that involve mixing of sexes (ikhtilat), music, and immodest display (tabarruj) are forbidden in Islamic law. If you have pledged to God not to attend them, then you must fulfill your pledge. Your husband does not have the right to force you to go to them in this case, and you should treat him well and explain the matter to him gently.

If the hall is free from forbidden things (munkarat), then attending it is originally permissible. However, your vow prevents you, and you may go and pay the expiation for an oath (kaffarat yamin). But if the celebration is at home or in a place that does not include forbidden things, then there is no harm in you going.