The son-in-law may have gotten some bad advice or may have misinterpreted advice he's received. He could also be trying to hijack the account.
As attorney-in-fact for his mother-in-law, he's not ordinarily in the position to make himself an owner of the account.
If SSA needs him to become a representative payee for his mother-in-law, they will want a separate account established, listing the lady as the owner, with her SSN on the account, and the SIL listed as Representative Payee.
Resist his attempt to make himself the account owner. Try to talk with the widow to find out if she has any idea of what's going on. Get you bank's counsel involved, if necessary.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8