You might wonder how fucked up you'd have to be to ship these two characters together, given the circumstances, and that would be a great question to ask Intelligent Systems, who made Rhea a completely real and valid romance option in the game. Meaning that this doujin is, in fact, almost completely canon-compliant, magical disappearing penises notwithstanding.

The lore made it better thank you

This is like way too late but another layer of this is that Rhea treats Byleth with a great deal of kindness, and Byleth thinks the world of her, but the reason she’s initially being so good to her is because she’s grooming her to either be a vessel for or sacrificed to bring back Sothis. So Rhea’s overwhelming thousand-year yearning to bring back her mother is running up against a growing guilt that fulfilling her wish will come at the cost of this person that trusts her and is intimate with her.