First, my condolences on your loss. With last minute beneficiary designations -- particularly where the change is allegedly made with the proceeds going to a stranger or "caregiver" -- there is a significant danger that your father may have been the victim of undue influence, or not realized what he signed, and fraud or forged signatures may be involved.
In almost any situation in which a life insurance company is faced with multiple claimants for the same policy proceeds -- no matter how suspicious its claims people may be -- the life insurance company will file an interpleader action and pay the proceeds into court.
As a former Assistant Genera Counsel of MetLife, who later retired as President and CEO of MetLife Brokerage, interpleader is the way we and other companies did it. No life insurance company wants to be placed in the situation of paying claimant A only to find itself sued by claimant B for the same proceeds, and risk a court concluding it had made a mistake. You likely would do the same thing if you were caught in the middle.
Of course, if both rival claimants can agree to an acceptable but less expensive alternative procedure than interpleader (such as arbitration) that will still hold the insurer harmless the insurance company may permit that.
The exact terms of the life insurance policy (and to a lesser extent the practices of the company) will typically determine what is required for a proposed attempt to change the beneficiary to be effective. In some cases the insurer merely requires that it have receive a signed written request from the policy owner before the date of the insured's death. In other cases the policy may provide that the notice be on a special form, or be received within a certain time frame, or even recorded or endorsed on the policy itself. In yet other cases, such as with some major employer provided group life insurance plans, the business providing the policy as a benefit to its employees maintains the records. It varies all over the lot. And there are a series of different standards that apply depending on the type of policy and whether it was covered by ERISA or not.
Leaving the emotional issues aside, if the amount of the policy is small I would urge people to try to resolve it with the other claimant before both of you spend more in legal bills than the face amount of the policy.
If the policy is for a significant amount, you'll need a lawyer to help, and I hope that you consider my law firm, Advocate Law Group P.C. We have offices in Northern and Southern California, I am licensed as an attorney in NY and CA, and we also work with other leading lawyers throughout the country. Our Sausalito office phone is 415-331-4350. And again, my condolences.
Sometimes these matters are simple to resolve, while at other times