Honestly, I wouldn't worry too much. It happens. I think people lose or break their NM pins all the time, they're usually not as secure or as sturdy as your actual badge. Just ask your chapter president (or whoever is in charge of the ordering of the badges) to order you an extra NM pin to replace your Sapphire sister's lost one the next time she places an order for badges/NM pins/guards. You can offer to pay if it makes you feel better, but they're so inexpensive that they might just tell you not to worry about it.
You also sounded a little bit worried that your Sapphire sister would be mad that it was HER pin you lost. Frankly, I don't know anyone who's too attached to their NM pin... I don't think it's all that big of a deal