A gratuitous promise, even if based on something being morally right, does not create a contract unless the beneficiary promises something else in return. Generally, such one way promises are not enforceable, although there are exceptions that are best discussed in person with an attorney.
As for the break-in, you can certainly file a police report and begin the investigation, but only a person appointed as executor or administrator of the estate can truly "press charges," take recovery of the property, or sue him over the thefts. If the brother is named as executor in the will, you should hire an attorney to assist in ensuring that the court does not permit him to serve.
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