Adda and her brother had an incestous relationship that resulted in pregnancy. This child was cursed (presumably either by Ostrit, who fell in love with Adda, or by their mother, who was furious about the incestous union between her son and daughter) and both mother and child died just after the birth. Foltest ordered that the two be buried in the Royal vault beneath his castle in Vizima.
In The Witcher computer game
The game suggests that it was Ostrit and not their mother who cast the curse, as evidenced by the contents of his journal.