there are many animes that would appeal to a 12 साल old but, many depend if the 12 साल old is a new ऐनीमे lover या a common ऐनीमे lover; i would suggest Naruto, Sailor Moon, Fairy tail, या Hunter x hunter
आप should probably go for shows where the 12 साल old girls are the target audience, for the most part.
I don't know your sister, of course, but here is what I would recommend to people of that demographic:
-Any magical girl ऐनीमे (Shugo Chara!, Sailor Moon, Pretty Cure, etc.) -Fullmetal Alchemist -Detective Conan -Any of the Big 3 (Bleach, Naruto, One Piece) -Ouran High School Host Club -Rozen Maiden -K-on! -Angel Beats -AnoHana -Inyuyasha
Really, it all boils down to what आप think your sister would enjoy the most.