It is usually very difficult to successfully contest a will. The objecting party has a heavy burden. It is necessary to prove (1) that the instrument was not executed properly or (2) the person who signed it lacked mental capacity to make a valid will or (3) the instrument was procured by fraud or undue influence. In order to have any chance of success in a will contest, the objecting party needs a very good probate litigation lawyer, and that is expensive.
Any opinions stated in response to Avvo questions are based upon the facts stated in the question. Responses to Avvo questions are for general information purposes only, and should not be construed or relied upon as legal advice.
I agree with my colleague. You would be required to establish sufficient facts to support the claim that your grandfather lacked testamentary intent or capacity, undue influence, fraud, duress, mistake or revocation. In Maine there are different standards of proof depending on the claim. For example, the burden of proof to establish lack of testamentary capacity is by a preponderance of the evidence whereas the burden of proof to establish fraud or duress is by clear and convincing evidence. In addition, the party who bears the burden of proof is different if the matter is litigated in the Probate Court or if the action is brought by way of civil suit in the Superior Court. You should consult with an experienced lawyer to determine the feasibility of pursuing this matter.