The situation that you paint is not all that uncommon. However, the facts including any medical reports on your dad will push the needle one way or the other. Being that his wife was the main recipient of his "generosity" makes proving things more difficult. Needless to say this is complicated.
If this is important to you I would suggest talking one (or several) attorneys to get their thoughts based on the facts that you have available. I would suggest finding a probate litigation attorney on this site or at naela.org.
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Go to your local law library. Look for West's Key Digest and look for your search terms. There should be similar cases listed under the same Key Numbers. The law librarian may be able to assist you with your search as well if you are having difficulty using the Digests.
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