According to the histories of John Skylitzes and Joannes Zonaras, he was extremely well educated, particularly in medical matters, and had been honoured with the office of magistros.
[1] He was elected to the patriarchate on 12 April 996, after the see had remained vacant for more than four years, due to the preoccupation of Emperor Basil II with his wars against the Bulgarians.
These regulations are soon after attested in juridical decisions but appear to have also caused some opposition (Logos Antirrhetikos of Skribas Nikolaos, c. 1030–1040).
[1] A later note in John Skylitzes' account also reports that Sisinnius II managed to finally lay to rest the quarrels of the tetragamy affair of Leo VI the Wise (r. 886–912).
[1] In 997–998, he also issued a typikon concerning the provision of the Metropolitan of Alania in the patriarchal monastery of Epiphanios in Kerasus.