A. Why go and upset her just for being generous? Aunts get a lot of pleasure out of spoiling their nieces and nephews, and this is one of the ways they do it. Look at it this way: Parents supply the necessities, and when they can, relatives supply the frills.
If you feel this present is inappropriate for reasons other than the price tag -- it's too delicate for a toddler, for instance -- then simply thank your sister for the gift, and whisk that delicate doll out of your child's slobbery little hands the minute she's gone (your daughter will probably be so busy playing with the box it came in, she won't notice it's missing!). Display it on a shelf in your child's room so Auntie will see that her gift is appreciated, and when your child is old enough to take proper care of her doll, let her play with it. Or you could just go ahead and let her run amok with love over it, and your sister can have the fun of giving her niece her first fancy doll.
This gift, and even years of other gifts like it, won't taint your child's values. She'll soak up the principles she sees you live by. If you establish family traditions that emphasize nonmaterialism, like making cards and little gifts for one another, and if you ooh and aah over a macaroni-glued picture frame as much as you do a pricey silk scarf, then your daughter will learn that when it comes to gift giving and receiving, it's the thought, not the price tag, that counts.