I am very sorry for your loss. You will wish to talk with an experienced lawyer in the jurisdiction where your father resided about the facts of this case and your potential for overturning this last-minute estate planning.
At the age of 18, very few people have the funds to pursue a difficult litigation. Nevertheless, some states allow attorneys to take a contingency fee in cases like this (that is, the lawyer, instead of earning a fee based on an hourly rate, takes a percentage of whatever you receive as part of the litigation).
Based on your location, I will assume that your father was a Texas resident. Unfortunately, I am not sure whether contingency fees are permissible in Texas probate litigations. Perhaps a Texas lawyer can address this issue in this forum.
Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
You should hire a lawyer to contest the will or file something in probate court right away if the will hasn't been offered. You present a case with interesting potential. The will is not automatically invalid but your Dad's medical records may establish that he lacked the ability to execute the will (testamentary capacity) or that he was weak and his relatives took control over his desires (undue influence). These type of cases are very fact specific. You need a lawyer in the area where the will contest will be filed, which is the County where your Dad resided at the date of his death. The lawyer can only decide whether to take the case after an extensive interview with you. The consultation/interview should be free of charge. Good luck to you.
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You should seek an attorney as soon as possible. You have stated a number of facts that would certainly make it worth hiring an attorney and challenging the will. There may very well be an attorney in the area who would be willing to either take the case on a contingency fee or require very little money up front, especially if your father had significant assets in his estate.