I did saw Nintendo just uploaded a video that has featured Dagr and Nott today. This also shows a flashback to what happened to the sisters' relationship three years ago and it turns out their mother had to let either the two compete each other who is worthy of the throne. Dagr tried to tell her mother didn't want to succeed the throne and let Nott take it, but their mother refused it. It looks like this is how it started the sisters complicated relationships in Book V.