K.P.
No I dont think its premature to be teaching her that she needs to be a little less "selfish" for lack of a better word. The sooner the better I say!
You did good though, because you did not relent, and buy her something too just because she was continuously asking for a present too.
Just be firm and tell her no, today is your sisters birthday, and not yours. When it is your birthday, then you get the toys etc, and she will not.
She cannot expect to get something every time, so good for you for standing strong. Its okay that you had to pick it out yourself, if she wants to pout I say let her for a little while, and maybe she will soon understand. She is still young however, so it will take a bit of time for her to fully understand the concept. But keep with it, and you will all be happier later.