disputing irrevocable beneficiary and misleading document

A brother had our Mothers' life insurance changed to him and checked a box on the form to make it irrevocable. Then sent the life insurance company a copy of a POA that we suspect was for a savings account that has been closed for 2 years. It was not notarized and is now null and void according to the bank. Some of the insurance company reps we have talked to stated they are recognizing it even though our sister has POA and her form is notarized and dated after his.He sent it over a year after the savings account was closed. Why he sent this in we have no clue, except to make the Ins Co. believe he is primary caretaker of Mom? Now if it can be proven that he sent in a POA that was canceled over a year before he sent it, couldn't this be conceived as misleading conduct, maybe a fraudulent act? He knew the account was closed and when. Could this be grounds to suspect he is not totally honest and require the Company to investigate the possibility that he also mislead the insured to signing the beneficiary change form and then checked the box making him irrevocable after she signed it. She has stated to other family members she doesn't want him to quote "get a dime of my money". The insurance company will not investigate and will not interview our Mother either. If we can prove a misleading act will they be required to investigate? We went to prepay our Mothers' funeral arrangements this past year and the Ins Co. thought she died, this is how we found out about the change of beneficiary. They sent us a letter in 2010 that the Ins was changed to our dad, who died in 1992. If the bene form was made out to a Sr and not a Jr, couldn't this be enough to require an investigation?