After the disappearance of her half-brother, the caliph al-Hakim bi-Amr Allah, in 1021, she was instrumental in securing the succession of her nephew Ali az-Zahir, and acted as the de facto ruler of the state until her death on 5 February 1023.
[8] It is still debated among modern scholars whether al-Sayyida al-Aziziyya was also the mother of Sitt al-Mulk's younger brother, and al-Aziz's heir and successor, al-Hakim bi-Amr Allah (r. 996–1021), but the evidence appears to be against it.
His half-sister Sitt al-Mulk therefore supported another candidate, an adult son of the prince Abdallah, who had been the first designated heir to Caliph al-Mu'izz li-Din Allah (r. 953–975) but had died shortly before him, leading to the accession of al-Aziz.
[28] The state suffered from an increasingly erratic governance, as the Caliph issued a bewildering array of prohibitions ranging from food and singing in public to dogs and baths, launched a persecution of Christians and Jews (culminating in the destruction of the Church of the Holy Sepulchre in 1009), purged the old guard of officials he had inherited from his father, and introduced doctrinal innovations, even to the point of seemingly accepting the divine status accorded to him by some of the Isma'ili faithful (who would later found the Druze sect).
[32][33] Sitt al-Mulk was committed to the succession of al-Hakim's surviving son Ali, and took him and his mother, the umm walad Ruqayya, into her palace to shield them from the Caliph.
[37][38] Modern scholars are likely divided, with some, such as Yaacov Lev and Fatema Mernissi, considering her participation as probable,[39][40] and others, such as Heinz Halm, considering it dubious,[41] since there were many other members of the Fatimid establishment, including Ibn Dawwas, who had an interest in eliminating the erratic caliph.
This was followed soon after by al-Hakim's designated heir, Abd al-Rahim ibn Ilyas, who was then serving as governor of Damascus: he was lured back to Egypt, imprisoned, and killed.
[45] The new caliph had long been under her tutelage during al-Hakim's reign, and remained so for some time after his coronation, which led to tensions with al-Zahir's mother, Ruqayya.
[46][47] She also reversed her brother's manifold prohibitions, allowing women to leave their homes, and permitting again the listening to music and the drinking of wine.
On the one hand, while women of the Fatimid dynasty were not usually involved in politics, they were "not secluded from social and economic life", which not only allowed them contact with the world outside the palace, but even necessitated the supervision of the administrative and financial agents acting on their behalf, giving them some experience in such matters.
Finally, particularly after the chaos and terror of al-Hakim's last years, the ruling elite was quite prepared to accept direction from her, for they had little left to lose.
As Lev writes, in this atmosphere, "machinations and the readiness to shed blood had become accepted ways of conducting political life, and by these standards, Sitt al-Mulk was a ruler worthy of the name, for she inspired hayba [i.e., awe]".
She may have de facto exercised the functions of a caliph, but it was unthinkable for her to hold power in her own name and claim sovereignty, for example by having her name included in the khuṭba (Friday sermon).
She also worked to reduce tensions with the Byzantine Empire over the possession of Aleppo, but before negotiations could be completed, Sitt al-Mulk died of dysentery on 5 February 1023 at the age of fifty-two.