The interesting thing is that the Nibelungenlied dispenses with the earlier aquaintance between Brünhild and Siegfried altogether. He still is complicit in tricking her into marrying Gunther by using his invisibilty, but he truly didn't know her before everyone went to Iceland, nor is he in love with her. (Thus there's also no need for a love/forgetting potion.) In Wagner's time, everyone assumed the Volsunga saga predates the Nibelungenlied, but as far as I know current literary theory is that it's actually the other way around.