The Surrogate's Court Procedure Act allows for a fiduciary to petition the court for permission to resign (Section 715). Since you have an estates attorney on retainer, you could ask her or him if you might make this application to the court.
Codicils generally require the same formalities as a complete Will to be effective. And with document assembly software, an attorney who concentrates in estate planning may be able to provide an updated Will for little additional cost. It's a good idea to have a Will reviewed every few years, and especially if a Will is over ten years old it is worth hiring an attorney.
In addition to the points already made, a Revocable Trust (also often known as a Living Trust) may be useful where a Will Contest is expected by a disinherited family member. Such a trust may be more difficult to attack than a Will. This is an option to discuss with an estate planning attorney.
Mr. Leonardi has given you a good overview of the issues involved. The hard news is that there is no duty to leave an inheritance to one's adult child. So, it really does depend on the factors outlined in his reply. To start, you should ask an estates attorney what your rights are regarding the status of your father's estate, and if there any issues with his Will (if any) which might be raised in a probate proceeding.
I would also add to the excellent assessment by Mr. Goralka, that you be on the lookout for any assertions that the Revocable Trust was revoked and whether the online will in question was executed with all the formalities for it to be legally effective. Online and DIY Wills are fraught with possibilities of failing to meet the necessary requirements.
This is a difficult situation. NYS law allows for a petition to be made to the court to appoint an "Article 81 Guardian" for an adult who is alleged to be incapacitated, either mentally, physically, or both and to a wide range of degree. You would have to consult with an attorney experienced in handling these guardianships to see if there is a chance it might apply to your Aunt's condition. Elder Law attorneys are often well versed in these matters.
As Mr. Potter notes, speak to an Elder Law attorney. Any transfers of assets may have an effect on eligibility for Medicaid and other benefits programs. In addition, NYS recently changed the laws on powers of attorney where gifts are involved, especially where the gift is to the agent under the power. It's very important for all involved to seek specific legal advice.
This area of law is especially state and even locally specific. Since estate recovery is a hot issue due to budget cutbacks, I suggest you consult with a Maine estates &/or elder law attorney, even though the estate is small. You might try a member of the National Academy of Elder Law Attorneys (NAELA) who handles estate recovery issues.
You could check with an attorney in the relevant state if there is a court proceeding to compel production of a Will. In NY, we have a proceeding under SCPA Section 1401, so you might see if there is a similar proceeding available. In addition, concealing a Will can be a very serious thing. Again, in NY, unlawfully concealing a Will can be a felony.
You'll need a lawyer to take a look at the Will and see if it meets the formalities of execution to be valid. Some local Bar Associations and law school clinical programs have events (sometimes monthly) where you can go in and have a document reviewed pro bono. You might check those in or near your county. It's worth calling to find out.