Fiona was also one of the three children known as the Houtborg triplets, a set of twins and another child, born while Riannon, queen of Temeria, was imprisoned by Falka. The other two children were Adela and Amavet. After Falka died, none, including Riannon, who had gone insane, knew which of the three children were her own twins and which was Falka's child. Sorcerers summoned by king Goidemar later found out that Adela was Falka's daughter, but they told the king that they weren't able to determine which child was Falka's, to avoid a scandal and the execution of a 2-year old child. Therefore the three were brought up as siblings.
Later information dug up by Fenn would suggest that Fiona was actually Falka's daughter, not Riannon's.