"Don't you know? He's a hero of the Völsunga Saga! A member of the Völsung family, son of King Sigmund and brother of Sigurd the Dragonslayer! Though he died young and journeyed to the underworld, rumours say his power rivalled the great Sigurd's! He's a fierce warrior who slaughtered the Siggeir family!"
Brother of Sigurd the Dragonslayer? That meant…
"Father's... older brother?"

Fate/Lost Einherjar, Chapter 2