I think that your intention sounds very sweet.

However, we all have different ideas as to what is age-appropriate for different gifts. While your niece sounds a little unusual to you, perhaps it is because she is aware of her mom's disapproval of what you've bought for her in the past? I assume that she may have heard how her mom thought that it wasn't the right time; or maybe it's something/s that she wants to provide for her daughter, herself.

I would recommend giving her a party at the Build a Bear workshop, make an afternoon of celebrating the holiday & her birthday, by allowing your neice to pick a friend to join you. Or, take her to an aquarium with a friend to celebrate. While you're there, she can pick out her favorite from the gift shop!

Just some ideas ~ special activities are special memories. I would recommend that you try to okay whatever your plan (or gift idea) is with her parents (primarily your sister) beforehand, just to smooth things over a bit.

I wish you the best of luck & hope that your love comes across clearly. Take care!