Halldóra Sigvaldadóttir was the daughter of Sigvalda Gunnarssonar langalífs, who had a secular position within the church.
She was the aunt of Gissur Einarsson, and is known to have supported his studies and career, which eventually resulted in him becoming Bishop of Skálholt in 1540.
As Bishop, he participated in the introduction of the Icelandic Reformation and thereby the dissolution of the monasteries, including that of his aunt.
Kirkjubæjar Abbey was declared dissolved and the property of the king and banned from accepting new novices, but the former nuns were allowed to remain in the building if they wished.
In 1544, six nuns remained in the convent, and Halldóra Sigvaldadóttir is noted still to have been in charge of them as abbess.