I'm not a Florida attorney, so I can't be sure there isn't a simpler way to do this.  But I'm pretty sure that if your grandmother makes a will, she can make a legally binding plan for her remains, and she has the right to name an executor in the will and give him or her the power to see to her cremation.  In many states, a person can also make a provision in their will that takes away all or some of what it gives to anyone who disputes the will.  Your grandmother should talk to a lawyer, and have her own wishes put into writing in the proper way.