This dilemma is all about the issue of responsibility. I find it ironic for starters, that it is you who wrote in the question, instead of your niece, who is clearly the one responsible for deciding what to do in this awful situation.
The fact that the stepmother and your niece do not have a close relationship is perhaps interesting, but not relevant. The stepmother trusted her with a special piece of jewelry. The state of their relationship has nothing to do with figuring out what the moral recourse is here. Nor is the fact that the jewelry was expensive. Again, it may be curious at best, but it doesn't really have anything to do with who's responsible.
When someone loans you anything, there is an unspoken code of ethics involved. The first consideration is to return the loaned item in the exact same condition it was in, prior to you borrowing it. At the very least, we expect the borrower to care for the item as though it was their own. In fact, we expect the person to care for the item even better than if it was their own!
At the same time, there are unforeseen circumstances that do occur, through little fault of our own. It is possible for instance, for a bracelet to get caught on something and at her own wedding reception, chances are the bride wouldn't have even felt it or heard it come off. So we should not be quick to judge, or jump to the conclusion that the bride was negligent in her care of the jewelry.
Nevertheless, the item was lost while in the bride's possession, and therefor she is the one who is responsible for resolving the issue to her stepmother's satisfaction. When we're talking about an expensive piece of jewelry, often the piece has been commissioned for that person only, which would make it impossible for it to be replaced exactly. In that instance, another arrangement must be worked out, to the stepmother's satisfaction (as opposed to a mutual satisfaction). When it comes to the recourse, the bride must be the one to initiate a conversation with her stepmother, express her deep apologies, and ask her what solution she would find acceptable.
Think about other situations when we become borrowers. Getting a loan from a bank, leasing a rental car for a day when your vehicle is being repaired, taking books out of the library, renting a video-- These are all instances when we are asked to sign formal documents attesting to our responsibility for the period of time we are borrowing something that is not ours to begin with. The reason your niece was not asked to sign a legal document, is because of the nature of the situation (her wedding) and fact that the two parties are family members. The informality of the transaction, however, leaves your niece no less responsible.