User blog comment:B214/Question and Answers 6/@comment-9595671-20160714212150/@comment-9595671-20160715002351

Ooh, my mistake there.

It's possible that Valkyrie traits can't be passed down to children, so if a Valkyrie has a daughter, that said daughter is a Valkyrie by default or needs to be trained to be a Valkyrie and be approved by Odin himself/

So Rossweisse was either always a Valkyrie, or became after training and Odin deemed she could become one.