Little girls of that age usually like role play by dressing up. There are many suitable outfits in the shops and Woolworth normally have a good selection. I bought my grandaughter a Stephanie outfit for her 3rd birthday and she loves it.
The Woolworth web address with dressing up clothes on is so long and would stretch the page too much but if you Google "Woolworth dressing up clothes" and then go into "Buy - Dress Up - Girl's Clothing - Baby Shop and Clothing", you will find them.
Early Learning Centres do doctors/nurses bags or cases and there are uniforms that go with these in various shops.
I also bought my grandaughter a lovely, illustrated nursery rhyme book and she adores it.
Caroline, I would have a word with her mother to find out what your niece would like and there would be less chance of her getting duplicate presents.