I realize this may involve more work on your part, but could you proactively try to steer her in the right direction? Find out from your brother what the kids like. Then, look through some sale flyers and find when something like that is on sale. "Oh, mom, did you see that sale on KidWidgets at BigStore? Yeah, they're really hot now. Bro said that GirlKid is really into them. If you haven't gotten a present for her yet, you should really go get one. I bet she'd LOVE it!"
Or if she's whining about their perceived lack of gratitude, just be blunt. "Mom, is the present for THEM or for YOU? If you want them to gush and be hopping-up-and-down excited, then you have to give them something that TO THEM is gush-worthy. Do you think they like to listen to Frank Sinatra or the Beatles just because YOU liked them? No. They probably like One Direction or Carly Rae Jepsen. So you have to think about what THEY like, otherwise just be happy that you're getting a polite thank you."