To be fair, I don't think Seteth is literally a child of Sothis in the same way that Rhea is. Meanwhile if you consider Sitri her daughter then Rhea is literally Byleth's grandmother on his mom's side of the family, related to him by blood on his father's side, and spiritually his daughter through Sothis.
But at the same time, Rhea is literally the daughter of a goddess and a basically immortal dragon that's been alive longer than pretty much everyone else so rules don't really apply to her. Also her support is wholesome as fuck.
Don't forget you can S-support Sothis, which in a way is just really self-involved masturbation.
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S*** I have to stop doing that," Gheb said, as he lay back down and died again. - Forgotten Love The Chiefs won the Super Bowl!