I'm sorry about the mess you have inherited. First, make sure you keep records and receipts for all estate-related expenses. In most states you should also be tracking your time as you may be able to be reimbursed for time spent, at least if you are appointed as executor.
While I practice only in Colorado, typically a will will only be disregarded if there is actual evidence that your uncle was not of sound mind and/or that you (or someone else) was unduly influencing or pressuring him. A mere allegation based on "belief" would not be enough. It will come down to the facts of the case and what evidence is presented. Good luck!
All the best,
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Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I hope that you are represented by an experienced probate litigator -- if you are not, I strongly advise you to hire a good one. (Judy Flynn in Hingham is terrific and practices in Brockton.)
Determining the chances of the success of your case depends on knowing the facts about the signing of the will and your uncle's health around that time. Nasty allegations are part of trying to get a will thrown out -- the question for you is can those allegations actually be proven? What did the attorney handling the estate plan know and when did he know it? Who, if anyone, was at the meetings between the attorney and the decedent? Was your uncle in good health or deeply demented? Were the terms of the Will that was signed consistent with wishes he had expressed over the course of the years?
In the meantime, document, document, document. Pay the bills and keep close tabs on what you're paying.
You should not just sit back and wait to see what happens. As Executor, you have a duty to defend the will, and the costs of that defense would be payable from the estate. Their allegations should probably be responded to with what is known as a motion to strike. If you don't take that step and succeed, you will be forced to go through a trial.
In the will contests I've handled, sometimes the parties have found middle ground and filed a will compromise rather than go through the costs of a full-blown will contest. The first step, however, is a motion to strike, which could knock out their case entirely.