Whether or not someone establishes an estate account or whether he is sharing the funds with his sis is not really a concern of the bank. The real problem is that the bank, with knowledge of the passing of the mother, is now on the hook to her actual estate for each and every check you have allowed to be deposited into this account since gaining that knowledge. If this is a joint account with the right of survivorship, the money that was in the account at the time of death became the son's regardless of any estate.
The bank should have flagged the account immediately to stop any future deposits with mom's name on it. You should probably give your legal counsel a call at this point.
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