Maybe she has a boyfriend/girlfriend - in which case, 'love' takes precedence over everying.

Very often our gifts are more about us, the givers, than about the person we gift them too.

It's hard when we go to the trouble to look for, decide on, purchase and wrap a gift we feel someone would/should like, but they don't think that much of it.

Being bitter now is not going to help you or her, or your sister.

She didn't react as you wanted but then again why did you choose to give such an expensive gift in the first place?  Was it because she really wanted them or because you really wanted her undying gratitude?

Next time, perhaps just set a limit that you feel you can afford and give only what you know the receiver will want and leave it at that.

Your niece is not on this earth to please you and your sister should not feel that you have bought something she could never afford and so should also be grateful to you.

Give because you love giving, not because you want anything out of it.