assigned to them by _Völsunga Saga_, which tells their story in full.

Sigmund, one of the ten sons of Volsung (who is himself of
miraculous birth) and the Wishmaiden Hlod, is one of the chosen
heroes of Odin. His twin-sister Signy is married against her will to
Siggeir, an hereditary enemy, and at the wedding-feast Odin enters
and thrusts a sword up to the hilt into the tree growing in the
middle of the hall. All try to draw it, but only the chosen Sigmund
succeeds. Siggeir, on returning to his own home with his unwilling
bride, invites her father and brothers to a feast. Though suspecting
treachery, they come, and are killed one after another, except Sigmund
who is secretly saved by his sister and hidden in the wood. She
meditates revenge, and as her two sons grow up to the age of ten,
she tests their courage, and finding it wanting makes Sigmund kill
both: the expected hero must be a Volsung through both parents. She
therefore visits Sigmund in disguise, and her third son, Sinfjötli,
is the child of the Volsung pair. At ten years old, she sends him to
live in the wood with Sigmund, who only knows him as Signy's son. For
years they live as wer-wolves in the wood, till the time comes for
vengeance. They set fire to Siggeir's hall; and Signy, after revealing
Sinfjötli's real parentage, goes back into the fire and dies there,
her vengeance achieved:

"I killed my children, because I thought them too weak to avenge our
father; Sinfjötli has a warrior's might because he is both son's son
and daughter's son to King Volsung. I have laboured to this end,
that King Siggeir should meet his death; I have so toiled for the
achieving of revenge that I am now on no condition fit for life. As
I lived by force with King Siggeir, of free will shall I die with him."