I go against your theory and I tell you why, in my opinion her the empress sister is the mother and the father is the emperor. Since it is hinted at several times that he cheated on the current empress and her mother ran away with the greatest treasure "Ellie". Which would also explain her golden hair and her green eyes are explained because of the mana stone her mother put into her.
She is, really unimpressed with novel and dropped it. Checked ratings today - still too high imo. Sigh - father is pathetic and never publicly acknowledged her even in end. Because he wants the ml as successor and ml/fl get together, so it was just kept quiet....trash imo
I think she is the duke's daughter.i think his wife was pregnant when the accident happened , and she didn't die just yet, the empress's sister save her and used the mana stone the green one on the duke's wife, and because she"the mother " is from the nation of the world tree it was easy , then the mother died and the empress's sister took care of Ellie.