Wait. Flayn and Byleth are related in someway right? Like distant relatives? If they are related then if you decide to marry her... isnt that like marrying your cousin or something? lol
I mean you can S-support Rhea on Silver Snow and she's the daughter of the spirit residing in Byleth's body, the genetic basis for Byleth's mother who was a homunculus, and gave blood to Jeralt many years ago to save his life when he protected her so she's related to Byleth in at least three different ways. It's best to not think about it too much.