Deja vu – I briefly dated a man in my 30's who gave me a dead, dessicated frog as a gift. I chose not to be offended; I collect natural souvenirs of all sorts myself, from dried leaves to jewel-like dead beetles, and use them in my art. Though I shuddered at this particular gift, I am quite sure he meant well, and the gift was "interesting" and scientific. But then we wanted the frog back a few months later to give to another friend. Hmmm – because that was one of many clueless behaviors, I realized I didn't want this man as a future step-dad for my daughter, so I stopped dating him. We remained friends, though, for a long time after.
Perhaps your sister really believed your mom would do this, and that you would appreciate the gesture. If you want graciousness in your relationship, it's really up to you to provide it. That would probably include giving her the benefit of the doubt.
What might be the most gracious thing to do is to just drop the issue, or, even better, be honest about your reaction (which you are entitled to, by the way!). You feel creeped out by the dead frog, in a Godiva box, no less? Well, your first impulse of disgust you most graciously suppressed. Good so far. But tearing down the gift, and your sister, to others is purely harmful to any future relationship, not only with the gifter, but with others who hear you complaining about her. Nobody is quite as comfortable with a gossip as with a person who is up-front in his/her communication. Most of us can never be sure quite where they stand with such a person.
The Golden Rule can be applied with grace or pettiness. If we behave toward others nicely because we believe doing this important thing can improve the quality of relationship for ours and future generations, that's one thing. But "being nice" because we hope to teach someone what we expect from them is something smaller, and converts gold into lead. We all do that sometimes, of course. It's a conundrum worth contemplating.
Do you really want to tell her how you feel about the gift? You CAN do this by sticking to FEELING words (I shuddered, felt nauseated, felt shocked/annoyed/horrified, my stomach clenched, I had to fight down a feeling of disgust, I was afraid to tell you, etc.), which are authentic, spontaneous reactions.
If you avoid JUDGING words (I felt insulted, unloved, offended; that was tasteless/clueless/ridiculous), which are the effect of your THOUGHTS about the gift), you'll avoid one of the most common causes of arguments. (Those who are interested can learn more at Non-Violent Communication sites.)
Everybody has thoughts about each other and other's behaviors, but greater maturity comes from recognizing that those thoughts are often rooted in our own individual ideas about what's appropriate, and why. Feelings, however, are common to all of us, and therefore understood on a basic level by all of us. They give us at least a chance at honest communication and understanding each other better.
Best of luck.