Executor trying to reclaim previously gifted jewelry, what can we do?

$50,000 is a lot of money, and I think you really need a lawyer to represent you here.

A simple gift like this isn't something that mom could have taken back now, if she were still alive, and I don't know what makes brother think he can win this.  The pictures are pretty good proof, and Brother will need something of some substance, to argue that mom didn't mean to make a gift and that she had just loaned the jewelry to your wife.

Your lawyer might consider asking the judge to either order brother to repay the estate, out of his own pocket, for the legal fees spent from the estate pursuing this -- or removing brother as the executor, if you think you want to take over the job.

As an executor your brother has a fiduciary duty to the estate and the beneficiaries. And it appears that he has breached this duty.

By way of explanation, a fiduciary duty is the highest standard of care imposed at law. A fiduciary is expected to put the best interests of the entity/person they owe the duty to above all else. This relationship is highlighted by good faith, loyalty and trust. Accordingly, a fiduciary must not put their personal interests before their duty, and must not profit from their position as a fiduciary. In your case, the fiduciary or executor is wasting estate assets by pursuing a frivolous lawsuit, as well as holding up the final settlement of the estate. He is doing so at your expense and his potential personal benefit. If the jewelry is considered an estate asset (which it won't be; it appears to have been lawfully gifted), he will benefit by getting a share that he would not otherwise be entitled to.

At this point, you need to contact an attorney about a counter-suit.  You should also notify the probate court of his breach and request his removal as executor.