It is not permissible for a woman believing in Allah and the Hereafter to observe mourning on the dead for more than three (days), except in case of her husband. (Using translation from Muslim 1491)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ «لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ وَالْيَوْمِ الْآخِرِ أَنْ تُحِدَّ عَلَى مَيِّتٍ فَوْقَ ثَلَاثٍ إِلَّا عَلَى زَوْجِهَا»