The executor has used estate funds for personal use. What can a beneficiary do?

If you were a beneficiary of an estate and you found out that the executor was skimming from the estate, would you know what to do? Would you know what legal solutions were available? A reader recently asked me what to do in that situation. His question and my response are below:

"My brother recently became executor for our mother's estate. He has asked all beneficiaries to sign a full release