Is it permissible for a wife to attend her sister’s wedding in a backless dress in the presence of women only, and is it permissible for the husband to allow her to do so?

The 'awrah (intimate parts) of a Muslim woman in front of other Muslim women is between the navel and the knee. If what the wife intends to reveal falls within this forbidden limit, then it is not permissible for her to do so, nor is it permissible for the husband to allow her to do so. If it falls within what is permissible, then there is no blame on her, and there is no objection to the husband allowing her, provided there is security against fitnah (temptation), photography, and the like. Moreover, a woman must observe modesty and concealment among women and avoid anything that compromises modesty or detracts from honor. It is advisable for the spouses to communicate and discuss matters wisely.