Authority letter to do legal work in court

Your nephew has to execute a Special Power of Attorney for the purpose of attending legal works only in your favour.

You can ask your nephew to execute a special power of attorney deed in your favor for the purpose of court work alone, duly notarised by a notary public of that country.

Then you can file a petitoon to permit you to represent your nephew in the case on the basis of the power of attorney deed.

What sort of Court work is involved ? 

You will be appearing on behalf of your nephew i.e., as complainant / plaintiff / appellant / petitioner/ accused / respondent / his attorney / counsel stated to have been located in America ?