Just because you are named as executor in a will does not mean you are required to serve as an executor. If your mother-in-law is still alive, you can inform her you no longer want the position, and she will need to amend her will to name a different executor. If she has already passed away, you can inform the successor executor, if there is one, or the court that you do not want to serve in this capacity. If the will is already being probated, you will need to formally resign in writing in the probate court and provide a written accounting of what you have done. If the will does not name a successor executor, the probate court will choose an executor after you resign. State law dictates who has priority to serve. The surviving spouse usually has first priority, followed by children. If there is no spouse or children, then other family members may be chosen. If more than one person has priority and the heirs can't agree on who should serve, then the court will choose.
For more information about an executor’s duties, go here: http://www.elderlawanswers.com/what-is-required-of-an-executor-6434.