It's a watercolor picture for her room that Auntie Liz painted herself. Will your daughter say the right thing after she rips off the wrapping paper? Frankly, you may need to prep her ahead of time, so seize the moment now and avoid some embarrassment--and possibly hurt feelings--on the big day. A few scenarios to consider:
She already has it. Nearly everything is returnable, so if the present turns out to be yet another copy of James and the Giant Peach, remind her to say "Thanks--I love this book!" and then the two of you can quietly exchange it for a different one later on.
She didn't ask for it. Her birthday wish list was composed entirely of items from the American Girl doll collection and Bendaroos, but your mother-in-law got her a chess set. Explain that grandparents try hard to please, so suggest she say "Awesome--this looks fun!" even if she's not sure she'll like it.
She hates clothes. There aren't many kids this age who will jump for joy at the sight of a new sundress or a pair of pajamas. Let her know that sometimes gifts are toys, while other presents may be items she needs. A polite "Thank you" is required either way.
She's gloating. Wallowing in the glory of a coveted Wii game while her dejected brother looks on is not an act of kindness; encourage her to tell her sib that she'll definitely share her prized present.