According to the Völsunga saga, Siggeir married Signy, the sister of Sigmund and the daughter of King Völsung.
[3] At the banquet Odin appears in disguise wearing a cape and a hood and sticks a sword in the tree Branstock.
Signy gave Siggeir two sons and when the oldest one was ten years old, she sent him to Sigmund to train him to avenge the Völsungs.
The boy did not stand a test of courage so Signy asked Sigmund to kill her worthless son.
[3] Signy came to Sigmund in the guise of a witch and she and her brother committed incest and had the son Sinfjotli.