First of all, she is not the one who decides if the Will is valid... the probate judge gets to do that. Secondly, she doesn't have the right to change the Will. With that said, she has every right to contest the Will, but she will need to hire an attorney and do it in court. Talk to the attorney who is handling the probate.
First, I am a NY attorney and cannot advise you as to your state's laws. I can, however, give you certain general information that may be helpful. If your mother did indeed wish to give her interest in the business to an employee, she was free to do so. Your posting suggests, however, that she did not do so. That is the beginning, the middle, and the end of this story.
That your mother may have misled certain persons as to what was in her will does not, as a legal matter, bind her in any way, with the possible exception that there may possibly be a claim in what is called promissory estoppel. The facts in your posting are not sufficiently detailed to allow any of us here on avvo to determine whether such a claim would be even arguably viable.
You should understand, however, that the only persons who can contest a will are persons who are either (a) named as a beneficiary in a prior will of the same decedent, or (b) a person who would be entitled to share in the estate if there were no will (such a person is known as a "distributee").
But let's call a spade a spade. One man's "threat" is another man's promise. Your mother may have threatened to disinherit various family members, but in doing so, it sounds as though she also made promises to certain persons. Whether such promises are actionable in any way cannot be determined by your posting.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as rendering legal advice involves the ability of the attorney to ask appropriate questions of the person seeking such advice and to thus gather appropriate information. In addition, an attorney/client relationship is formed only by specific agreement. The purpose of this answer is to provide the questioner with general information, not to outline specific legal rights and remedies.