If your sister passes and has no children and her husband is not living and her nephew has POA, what happens?

First of all, a POA ends upon the death of the principal, here your sister. So the nephew, as agent, has no legal rights at this point. Next, when someone does without a Will they are said to have died "intestate". This means that the intestacy laws of the state in which they were domicled when they died will control. Typically, in such a case, the heirs would be the surviving spouse, if any, and the children of the deceased. Since there are neither in this case, then the next heir iwould be the deceased's parents. If your parents are not alive, then the heirs next in line would be sibiligs, you and your brothers/sisters, if any. At this point, you can go to the probate court in the county in whci your sister was domiviled when she passed, and asked to be appointed the "personal Representaive", which is the equivalent of an executor when there is no Will. If appointed, you will be in charge of adminstering the estate. This means that you will need to inventory your sister's assets, make sure that any creditors are paid, and then distribute the remaining funds, etc. to the legal heirs.