It is not permissible for a woman believing in Allah and the Hereafter (or believing in Allah and His Messenger) that she should observe mourning for the dead beyond three days except in case of her husband. (Using translation from Muslim 1490a)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ وَالْيَوْمِ الْآخِرِ أَوْ تُؤْمِنُ بِاللهِ وَرَسُولِهِ أَنْ تُحِدَّ عَلَى مَيِّتٍ فَوْقَ ثَلَاثَةِ أَيَّامٍ إِلَّا عَلَى زَوْجِهَا