Assume that a lawyer forges his or her client's signature on a will.
What is the remedy for anyone who believes they've been injured by the execution of the will?
You can ask a law enforcement officer or district attorney to press charges. Generally, you cannot do so yourself. In practice, this kind of allegation is almost never criminally prosecuted. A prosecution would be particularly unlikely if the witnesses and/or notary confirmed his story that it was your father's signature or was authorized, and if the lawyer received no personal benefit from the will.
Sentences for forging a will vary from state to state and sometimes depend upon the amount of personal benefit secured from doing so. Absent a huge personal benefit, it would probably be a minor felony punishable by a few years in prison or more likely, in the case of a lawyer without prior convictions, a deferred judgment, or probation and a fine.
Proving that the signature is forged is extremely difficult and under certain circumstances a person trying to execute a will can lawfully have someone do it for them by directing the lawyer to do so in the presence of the witnesses (for example, if your father's hand was shaking too much to make it possible for him to sign his name due to a condition like Parkinson's disease). The witnesses to the will would be in a bad position to testify against him because they could face charges too if they witnessed the will despite knowing that it was not signed or authorized by the testator.
You could contest the validity of the will due to an allegedly forged signature by raising a timely objection in a probate proceeding to have the will determined valid. In this proceeding, you would only have to show that it is more likely than not that this happened and the consequence would be that the will would be found invalid.
In theory, you could sue the lawyer for damages to you caused by the alleged forgery, but only if you were harmed and realistically, only if the will was found to be invalid, which would eliminate any harm, unless the probate process has already been completed.
Finally, you could complain to the attorney regulatory body in the state where the lawyer practices. If they found your allegations to be credible they would investigate. If they found that your allegations were probably true following an investigation and a hearing, they could suspend his license or disbar him.
Probate law is complex, and varies by jurisdiction, but in principle a will with a forged signature should be set aside. In England and Wales, anyone who has legal standing can issue a claim that the estate is not being properly administered; if they prove that it is being administered on the basis of a forged will, it seems likely that they will win. In that case, the lawyer might face criminal charges (depending on whether the Crown Prosecution Service thinks it worthwhile) and would probably be struck off by the Law Society; he would certainly have to pay the costs of the improper administration (and, of course, the court case).
However, somebody still has to administer the estate; the court would probably treat it as a case of intestacy.