She was the posthumous daughter of Roger II of Sicily by his third wife Beatrice of Rethel.
Constance was not betrothed until she was 30, which is unusual for a princess whose marriage was an important bargaining chip. This later gave rise to stories that she had become a nun and required papal dispensation to forsake her vows and marry, or that she was impossibly ugly. Neither of these is consistent with the evidence.
The death of her nephew Henry of Capua in 1172 made Constance heiress presumptive to the Sicilian crown. Her elder nephew King William II was himself unmarried. He did not marry until 1177, and that marriage remained childless.
But it is unclear why he delayed so in finding a husband for his aunt. Nor it is clear precisely why in the end he chose Henry, the son and heir of the Holy Roman Emperor Frederick I.
The Sicilian kings and the German Emperors had long been enemies, and the papacy, also an enemy of the emperors, would not want to see the great kingdom to the south of Rome in German hands. Nor would the kingdom's nobles welcome such a possibility.
Nevertheless, in 1184 Constance was betrothed to Henry (the future Emperor Henry VI), and they were married two years later, on January 27, 1186.
William made his nobles and the important men of his court promise to recognize Constance's succession if he died without direct heirs. But after his unexpected death in 1189, his cousin (and Constance's grand-nephew) Tancred seized the throne. Tancred was illegitimate, but he had the support of most of the great men of the kingdom.
Constance's father-in-law died in 1190, and the following year Henry and Constance were crowned Emperor and Empress. Henry was already preparing to invade Sicily when Tancred died in 1194. Later that year he moved south, deposed Tancred's young son William III, and had himself crowned instead.
While Henry moved quickly south with his army, Constance followed at a slower pace, for she was pregnant. On December 26, the day after Henry's crowning at Palermo, she gave birth to a son, Frederick (the future Emperor and king of Sicily Frederick II) in the small town of Jesi, near Ancona.
Constance was 40, and she knew that many would question whether the child was really hers. Thus she had the baby in a pavilion tent in the market square of the town, and invited the town matrons to witness the birth. A few days later she returned to the town square and publicly breast-fed the infant.
Henry died in 1197, and Constance returned to Sicily. She had the 3-year-old Frederick crowned King of Sicily, and in his name dissolved the ties her late husband had created between the government of Sicily and of the Empire. She also renounced his claims to the German kingship and empire, and placed him under the protection of pope Innocent III. She expected him to be raised as a Sicilian, and to be nothing more than King of Sicily. That he became much more than that could not be predicted when she died a year later, in 1198.
Dante places Constance in Paradise (though he subscribed the story that Constance had been a nun):
"This other radiance that shows itself
to you at my right hand, a brightness kindled
by all the light that fills our heaven-- she
has understood what I have said: she was
a sister, and from her head, too, by force,
the shadow of the sacred veil was taken.
But though she had been turned back to the world
against her will, against all honest practice,
the veil upon her heart was never loosed.
This is the splendor of the great Costanza,
who from the Swabians' second gust engendered
the one who was their third and final power."
(Paradiso, Canto III, lines 109-120, Mandelbaum translation)