Is it permissible for a woman to trim her mustache and wear black during her waiting period (iddah) following the death of her husband's brother?

It is not permissible for a woman to mourn for a deceased person for more than three nights, except for her husband, for whom she mourns for four months and ten days, as is authentically established in the two Sahih collections and other works. Based on this, it is not permissible for a woman to mourn for her husband's brother, nor to abandon adornment for more than three days. As for wearing black for a woman observing the 'iddah (waiting period) of death, there is no legal basis for it, and it is prohibited if it is considered an adornment in her environment. A woman is not permitted to mourn for any of her relatives and abandon adornment for more than three days, except for her husband.