You need to have a lawyer look at this, right away. There are some major red flags. It is not clear how the Wills were done and redone, but my strong guess is that this was NOT done through an attorney. Given your grandfather's age and illness, his capacity is certainly in question and he also could have been subject to undue influence. Wills are given deference by the courts, so the burden of proof is against you and it will not be a difficult case, by any means. In order to prevail, you will need the assistance of a very skilled probate litigation attorney. You should seek advice from such a person, as soon as possible.
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In order for a will to be valid, Texas law requires an individual to have testamentary capacity or be of "sound mind" at the time he executes his will. If your grandfather had dementia at the time he changed his will, there may, at the very least, be questions about his capacity to make a new will. The facts you provide raise a number of other possible concerns as well. It would be to your advantage to consult with an attorney who is experienced in probate litigation.
If your grandfather was diagnosed with dementia then he may not have had the mental ability to re write his will. You should contact an estate planning attorney right away so that you can best explore your options.
The information is not, nor is it intended to supplant legal advice. The reader should not consider this information to be an invitation for an attorney-client relationship and should always seek the advice of a qualified estate-planning attorney regarding your own situation.