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I am sorry to hear about this situation. On this website, I do not always get to give good news, and this is one of these times.
She can attempt to turn in a bill. However, the Executor of the estate (or another heir) can try to block it, and if so, then they may be successful. Without an actual agreement in place in writing, the Court would presume that all her work was a gift and volunteering for your mother in law by the daughter.
She can try to argue that there was a verbal agreement, but per MN 513.01(1), any agreement for something over a year has to be in writing, so her argument would be weak since there is nothing in writing.
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