Kids are more spoiled than ever. We give them so many toys and gadgets that they wind up not appreciating any of them. Instead of asking why all their Barbie dolls are naked, we should be asking why they have more 5 of them to begin with. Is there going to be a Barbie convention? Are they forming a band? No. It's overkill. Stop it already.
Look, it isn't you. It's them. The rest of the gift-buying family. I'm sure they mean well.
When they ask what she likes for her birthday, don't just go to the well and say "anything pink or princess." That just makes it worse. Tell them to buy books (these are paper-based readers that contain stories.) That way you avoid getting 5 Barbies, 3 copies of the same Disney movie that you already have saved in your DVR for emergencies, and a pink outfit that looks ridiculous but instantly becomes her favorite.
If they do get new toys, guess where the old ones go? Donation. Have them help you go through their old stuff and explain that it's time to share with other kids who don't have as much. You'll be surprised how much they want to help once they understand why.
And no, child. You cannot play with my iPhone. It's not a toy. Daddy's reading the paper.