Amelia is Mickey old sister, chased away and cursed by Mircea (their father) because she was a female and, with Mickey's birth the cruel voivoda throw a curse on the daughter, condemning her to travel restless in the world and never return at the castle.
Amelia and Mickey will meet again in Frollo's dungeons, and she will do all she can for protect her brother.
She've almost the same Mickey's powers, plus she can create an intense darkness, that can obscure or extinguish every fire, or protect herself from the sunlight
P.S. for who don't know who's Amelia:[link]