Personally, I had my doubts about Rhea after she had all the Western Church followers executed, and after reading Jeralt's diary, joining up with Edelgard(my female Byleth's eventual wife) was a fairly easy choice.
All I said was true except the to have sex with her part that was a joke. But yeah rhea's obsession with sothis is on the crazy yandere girlfriend side rather than the daughter wanting her mother back