Vikings left a couple of questions unanswered and the biggest one is about Bjorn Ironside and his father. Is it Ragnar or his brother, Rollo? Throughout its six seasons, Vikings left clues pointing at both, but it’s most likely that Rollo is Bjorn’s biological father. Vikings initially followed Ragnar Lothbrok (Travis Fimmel) alongside his Viking brothers, one of whom was his brother, Rollo (Clive Standen). As the series progressed, it changed its focus to Ragnar’s sons – Bjorn, Ubbe, Hvitserk, Sigurd, and Ivar – and their journeys.
https://screenrant.com/vikings-show-bjorn-dad-rollo-not-ragnar-reason/