"...it's just a matter of which sister is the mother of Mordred which is at issue."

Which is exactly my objection (though it is a mild one). Very generally, when Mordred's mother is called "Morgan", it is because the family tree has been truncated (and mentioning other sisters while leaving them undefined is just covert truncation).

On the other hand... There are good dramatic reasons for Mordred's mother to be a strong character, and a powerful sorceress, in her own right, though it DOES shift the flavor of the story quite a bit.