Hoow can we make sure that my elderly gandfather is not talked into making a new Will by my manipulative cousin?

I am sorry your family is facing this situation.  Unfortunately, it is not rare, and it is not just family members who attempt to influence vulnerable seniors.

The only absolute way to keep your cousin from influencing your grandfather into signing a new will would be to establish a legal guardianship for him.  This is an extreme move.  It would require filing a petition in the Probate Court, having him examined by professionals, declaring him incapacitated, and terminating all of his rights to manage his own affairs.  In Florida, and I suspect in all other states, your mother's rights under the power of attorney would be suspended until the Probate Court says she can continue or appoints a legal guardian.  Any will signed during this time could be invalidated by the Probate Court and would be legally invalid if the court found your grandfather lacked capacity.

There are less extreme, but not foolproof, methods to stop your cousin.  You might try some of the less extreme methods while keeping in mind that your mother could hire a lawyer and petition for guardianship if they did not work.

Remember, first, that a will must be signed with specific formalities.  These vary a little from state to state.  In Florida, it must be signed at the end by the testator (your grandfather), in the presence of two witnesses, who also sign at the end.  The testator must declare the document to be his last will and testament, and the witnesses must attest to the fact that the testator is of sound mind sufficient to make a will (in general, this means that the testator knows what he is signing, knows who are the "natural objects of his bounty," and knows the will expresses his desires). 

Florida allows a will to be "self-proving."  In addition to the above, this requires a statement that bascially recites the requirements above, a second signature from the testator and each witness, and a notary's acknowledgment.  If a will is self-proving, it is not necessary to call the witnesses to testify to the validity of the will later when it is admitted to probate.  If the will is not self-proving, it will be necessary to call at least one of the witnesses (in Florida).

So, the first impediment your cousin faces is the need to have the required number of witnesses to the will.  Your cousin probably is not qualified to be a witness, so he would have to find two disinterested persons.  He might fail at that.  Your first, less intrusive means, is to prevent your cousin from taking your grandfather somewhere that he could set up this process.

Even if your cousin were to succeed in getting your grandfather alone with two witnesses and all of them were willing to go along with this, the will could be invalidated.  What you are describing is called "undue influence" and a will signed as a result of undue influence is invalid.  If your cousin got so far as to have your grandfather sign the will with the requisite formalities and attempted to probate that will, your mother and the rest of the family could challenge the will.  Keep notes, records, copies of documents, etc., in case this becomes necessary.

The second, less intrusive means, of stopping your cousin is to explain what I just said about undue influence.  Tell him the will won't be valid whatever he does and the family is willing to prove it.  Tell him it will cost him a fortune to try it, and he will fail.

Then there is the third, less intrusive on your grandfather but pretty awful on your cousin, means of stopping him.  Florida has special statutes that protect seniors from abuse, neglect, and exploitation.  I bet other states have similar statutes.  Taking advantage of a senior's diminished capacity to steal their money is exploitation.  If there are no special statutes for seniors in your state, this is still theft or attempted theft, a crime.  You could threaten to, and if that did not work, call the adult protective authorities, police, or prosecutor's office about your cousin's efforts.  In Florida, the adult protective authorities (1-800-96 ABUSE for those in Florida) will investigate.

I am not saying the authorities will take care of this for you.  Often, they don't.  They have their own standards for evidence and when they will take official action and when they won't.  It is often frustrating for families that authorities don't know what they know, believe what they believe, and act as they think they should.  You have to understand that they are third-parties who know nothing about your family.  They have to follow state rules and rules of evidence.  They can't act unless they have enough evidence.  Families and friends must remain involved to protect their senior loved one.

Several good things come out of calling the authorities.  If nothing else, you make an official record of your concerns.  The authorities are likely to contact your cousin, making him aware that law enforcement authorities are watching. If the authorities don't act on the first complaint and your cousin continues his efforts, call them again.  And again.  And again, if necessary. 

There are people who will continue their efforts to steal from vulnerable seniors no matter what obstacles stand in their way.  I hope your cousin is not one of these people and my suggestions help your grandfather.