My sister was like that. If she didn't have something, NO-ONE was allowed it, be it chocolate, a compliment, cuddles... or words beginning with the rest of the alphabet.

Speaking as the elder sister - nip it in the bud now. It will come to no good. Let the eldest know that she's special too. It's not all about the younger because she makes the most fuss. Calmly accepting things and being loving is WONDERFUL and needs appreciation.