User blog comment:IamJakuhoRaikoben/Chapter 459 & 460: Weakness & The Alighted Pegasus, Review./@comment-77.194.131.146-20151101102709/@comment-7418318-20151101104031

Gemeni can only copy being of similar or less magic power to Lucy. I doubt that a Star Dress could alter that rule, at least not to such a degree as to bridge the gigantic gap in power levels between Lucy and Brandish.