Stepmom changed Dads will and made self Executor when he’s not able to form sentences

I am sorry to hear of your dad's condition and of the possible manipulation by your stepmother.   As for your question, however: you can't prevent your father from changing his Will, even if you think your stepmother is effectively forcing him to follow her wishes rather than his own. What you MAY be able to do includes the following options:   1. If you think he is truly incompetent and that your stepmother is committing physical, emotional, and/or financial abuse with regard to him or neglecting his welfare, then you (or someone else) may be able to seek appointment as his guardian and conservator while he is still living. If a court agrees that he is incapacitated and in need of a guardian or conservator, then the appointment of that person can help make sure that he is being care for appropriately and that his assets are being used for his benefit and not wasted, stolen, or used inappropriately.   2. If he dies and you do not believe that any Will that is offered for probate for his estate is not a valid expression of his wishes (because someone exercised undue influence over him, the Will was not correctly executed, or because he did not have the level of competency necessary to make the Will), then you can try to challenge the Will. It's more difficult to do, but you might also be able to challenge beneficiary designations, trusts, or rights of survivorship that result in your dad's assets being transferred to others outside of his Will if you think that those were created under circumstances that make them invalid.   Either of these options means that you need to hire an attorney who does fiduciary litigation and handles contested guardianships/conservatorships or contested estates. You should likely assume that there will be an expensive, lengthy legal battle, and you should not assume that there will be any financial reward in it for you. However, if you are really concerned about your father's welfare, then you should look into option 1 and perhaps pursue it- that's the best way to protect him while he is still living, and it may also help ensure that his eventual post-death wishes come to pass.   Best wishes to you and your famiily.