Think is, their only established relationship is the implication that Odin made the valkyries from a Velber (so Altera is like their big sister)
Siegfried is the one that said his wife eventually married Altera/Attila
In the end left it ambiguous is their way of trying to differenciate between Siegfried and Sigurd, as well not explaining too much how Odin could still have children post-Nasuverse Ragnarok (1000 BC), since Sigurd is still established as his descendant