What should I do as an executor of my mothers estate when I have discovered funds missing and my brother had power of attorney?

I think the answer has to do with several factors. How much money is missing and was it spent of the thousands of little things you purchase for a parent who is ill or who is in the hospital? Also who had the main responsibility for your mother's care? Last of all, do you want to exist with some type of peaceful relationship with a sibling or does it matter to you? My sister-in-law still owes us 5,000 dollars from the final division of my mother-in-laws estate. She has 5 children;we have one. We love her and her children as well as her husband. We have dropped the matter. Is is fair or right? No, but it is what my husband's mother would have wanted. We can live in peace because of that.