hence the last name volsungr lol
been some time since i read any version of it but small correction:
sigmund is the father of sigurd/siegfried, not the other way around, he dies and his wife makes sure sigurd gets the sword gram
it's sigurd/siegfried who in some versions was the fiancé/lover of brunhild, not sigmund
but in some brunhild and gunnar/gunther have a son named siegfried too i think lol

I just realized, the main characters in this story are all inspired by characters from a story from Norse Mythology, the Volsung Saga. ML is Sigmund, the Hero who pulled out the sword Gram, MC is Brunhild , who is portrayed as his wife/love interest in some iterations of the story. ML’s Nephew is Siegfried, the replacement for Sigmund in the German version of the story, who was often said to be Sigurd’s son.