Hi E.,
You've certainly received a lot of thoughtful responses and there's so much here for you to digest. I'll try not to repeat what others have said, but will give you a few more opinions...
--Consistency and follow through are important. Even very young children are quick to catch on to parents who make idle threats. If you said you would be returning some presents to the store, then I agree that you should take her with you and return one or two presents (maybe not the one she was looking forward to the most, though). Before and after making the return, be sure you talk about it a lot so that she understands why you did it (maybe talk about the cost of replacing the shades) and be sure she KNOWS that you still love her and forgive her!! Also, since she is so young and this is probably going to be a difficult punishment for her, perhaps you could negotiate with her a way for her to earn back those toys that you've returned.
--I strongly disagree with the suggestions of donating her toys as part of the punishment. Returning 1 or 2 to the store is one thing, but don't donate them. You want to teach her to give for the joy of it. You do not want to equate giving to others with punishment and negative feelings. If you donate as a punishment, what happens when you want her to donate because she has too much and you want to teach her to share her "wealth" with others? IMO, she'll be feeling like she needs to hoard and protect her stuff and view giving up her stuff as another loss (like another punishment).
--Since you have not already told her that half her toys are going to the Salvation Army, I would forget about that entirely and let her start earning her toys back. Standing your ground and following through with consequences are important, but as others have said, she's probably too young for the delayed consequences. Also, if you get into the habit of giving huge, jaw dropping consequences for every infraction, you're going to be out of "ammunition" when the really big offenses occur (even though this seems like a big offense, think about it...it could be a lot worse, and as someone else said, she MAY have been coming from a perspective of boredom / curiosity, etc. and not sheer defiance).
-- You catch a lot more flies with honey than with vinegar. IMO, you've made your point with the punishments, but you also need to try to catch her being good and praise and reward her good behavior (positive reinforcement works better for most adults, as well). If she's feeling like she needs attention, she'll seek it any way she can, even if it's negative attention. Let her see that she can get positive attention by making good choices.
Ultimately, you have to do what you think is right. Don't worry about your sister-in-law's opinion. Good luck!