According to previous films, Dom raised his sister after their father’s death.
It’s unclear if the John Cena character was already out of the picture at that point, or if Dom tried to raise them both, but ultimately failed in some way.
While the Fast & Furious movies have never needed to be all that serious or realistic, the fact that the main character has a sibling that has never been mentioned in eight previous films, despite the fact that another sibling has been a big part of the films, certainly does feel like a major swerve in the story.