Can he authorize his daughter to sign some checks when she is not on the bank signature card?

Who is he? Technically, another person who is not on a checking account cannot sign someone else's name. That would be fraud/forgery unless the signing person had a valid power of attorney for the other person. If so, then the person should sign as illustrated by the following example "John Doe, as POA for Jane Roe" or something similar. The bank should also have a copy of the power of attorney on file. Of course, if this was done and the principal may have been too weak to write and authorized this and there was no one to object then the bank might pay the check. However, a lot of this is fact-specific. Who was the check made out to? What was it for? To pay the uncle's bills? Or was it to the cousin and made out fraudulently to steal from the uncle? The answers will indicate whether it was fraud or no. Now that the uncle has died, any money remaining in the account at the time of death, if the account was a POD (payable on death) account would go to the named designee. That does not excuse any fraudulent check writing that occurred before uncle died. The personal representative of your uncle's estate needs to review the checks that are believed to be fraudulent and see how much money we are talking about. If its financially worthwhile, the person who wrote the checks can be sued civilly and maybe even prosecuted criminally. If the person still has the money or the money can be traced to other assets (like a new car, house or investment account) then a constructive trust can be imposed and that money returned to your uncle's estate for distribution to your uncle's heirs as per his will or under the intestacy laws. If the money would end up going back to the person who wrote the checks, then nothing would be gained by suing though. You should discuss this with the probate attorney who is representing the estate of your uncle. If there is no attorney then the personal representative needs to get one.