16 Pa. Stat. § 4421
The district attorney shall designate one of such assistants as his first assistant. Such first assistant shall, in the absence of the district attorney from the jurisdiction or during his disability to perform the duties of his office through sickness or other cause, be vested with all the duties, powers and privileges given by law to the district attorney, and generally, at such time, be empowered to do and perform all things in connection with his office which the district attorney may by law be entitled to do or perform. In case of any such incapacity of the district attorney or his first assistant, or both, any or all of such duties, powers and privileges may be done by such other assistant district attorneys, if any, as may be designated by the district attorney.
16 P.S. § 4421