Wondering if a will can be changed if father was diagnosed with dementia?

My sister has guardianship over my Dad that lives in an assisted living home. Before she became guardian my ex-stepmother had a will drawn up which gave her son from a previous marriage executor of the will. They have been divorced for a few years now and the step-brother is no longer in the picture. My Dad doesn't have much but we want to be able to plan for his funeral not him. He lives in Texas.