...though I'm a virgin, I'm not promised to her. She's the equivalent to my great-grandmother.

[If one were to follow conventional family trees and sort of ignore how the method of procreation from Sothis to Rhea and Rhea to Sitri were... not how humans conceived children, then yes, Byleth was essentially Sothis's descendant.]

In any case, following your definition of 'god', then yes, she's a god. She created the people of Fódlan and the divine beasts, and has a religion devoted to her, though I don't know much about it. Also, our souls are tied as she no longer has an earthly body, so she rests in mine instead.

[While he normally omitted the whole "I have a goddess inside of me" with most people, it was usually because the topic never came up, and because he was aware - even with his poor social skills - that he'd sound utterly insane. Set was a god, however, and was taking the whole thing in stride, so Byleth decided to be blunt and straightforward in turn.

Besides, Byleth knew one of his deep dark secrets, being married to Manon. It was only right he'd give one in exchange.]