How Long Does Agent Under a POA Need to Keep Documents?

There is no hard and fast answer to your question. For tax purposes, people often recommend keeping seven years of records. For Medicaid purposes five years is enough, as you are doing. In terms of your siblings, it would be best to ask them. These days most of the information is available online, so you shouldn't need to save more than seven years of records -- and five years should be fine. If your siblings want more, perhaps you could send the statements that are more than five years old to them to review before they discard them if they choose.