What is the ruling on a woman attending weddings where it is highly probable that there will be reprehensible acts, given her inability to forbid them, and does the ruling differ if the marriage is for a relative?

If a woman becomes aware of a reprehensible act (munkar) and is unable to change it, it is not permissible for her to attend the gathering unless she can be in a place where she neither sees nor hears the reprehensible act. This ruling applies whether the gathering is for relatives or others.

Regarding the 'awrah (intimate parts) of a woman in front of another woman, the more cautious opinion is that the 'awrah is what typically appears from the places of adornment, similar to what a woman displays in front of her maharim (unmarriageable kin).