Question: how is byleth related to rhea?
Answer: Sothis is the spirit of Rhea's mother, who she desperately wants to see again. Partway through the story, you and Sothis merge souls. ... Well, so since Rhea is actually Seiros who's Sothis' daughter and Byleth is a reincarnation/soul binded with Sothis that would make her Byleth's daughter.