Anyone can name anyone as executor of the will. They can also write a new will that will over ride any prior or existing wills. These wills may or may not be recorded with the State or County in which the person resides. Your wife would not be automatic to be the next in line to be the executor/tris of the estate. This would have been determined by her mother unless your wife already had paperwork drawn up to this effect. If so then it would change matters. And wills are indeed normally sealed or retained by someone known to the person making the will. POA or Power of Attorney is what you need to find out if the current Executor has as this would allow her/him to sign and prepare documents. It would also allow them to access funds in accounts or CD's. Being an executor of a will does not make one the POA. They have to be noted as the POA. or Durable POA. There is something known as a spring POA as well. Although written for New York the terms would still apply for you.

http://www.oag.state.ny.us/seniors/pwrat.html

You need to find out if the person you believe has done this also has a Power of Attorney. If your wife has a signed POA from her mother making her the POA then the other family member needs to produce any document to the contrary.

Let me know if I may further assist you in this.