My nephew came in help to clear up my Mom's finances. I asked him to show me all documents. What can I do legally if he refuses?

Pam, the death in 2003 threw me off as well, then I realized that there's no real indication when the nephew began his asset reorganization plan. If it had been in 2003, it's pretty late to be concerned about it now.

Archangel, can you clarify? When was your nephew asked to help? When did he start?

Do you and/or sis have POA? If you do you can ask the investment companies etc for the statements, but I suspect that nephew has the POA or he would not have been able to change her investments or take out a mortgage in her name. If nephew has been appointed POA he doesn't have to tell you anything.
It might be best if you present this as a fact finding mission to go over your mom's future care needs. Plan a meeting with you and your sister (his mom?), it will seem less judgmental if you are all working together to assess her financial ability to pay for care.

All the documents for the last 13 years? You get a lawyer and you petition the court for Guardianship, but I doubt if they would go back beyond 5 years.
If mom signed a document making him POA, he is actually obliged NOT to share information, he has to keep it confidential. Is he POA?

I don't understand....he couldn't make the changes without your mother's concurrence, signatures or if he was proxy under a POA. Was he?

Who signed for the new investment accounts? Who signed the mortgage? Your mother?