One can't just take it upon themselves to do a tax return for another, even if they're deceased. Things have to be worked through the courts and a personal representative has to be legally recognized or appointed by the court. Then, and only then, can the personal representative do things such as file tax returns on the behalf of the deceased.
Even if you Uncle left a Last Will & Testament appointing you as his representative or administrator of his estate, that document has to be filed with the courts and run through the legal probate process before you or anyone else can act on the instructions of the will.
