Q: In other words, he only has relatives by marriage such as nieces, nephews and their offspring. Would these be valid heirs according to the intestate succession law?

A: The PA intestate succession statute, http://www.legis.pa.us/WU01/LI/CT/HTM/, comprehensively covers the lines of familial lineage. It dictates who inherits from someone who dies without a will. A reading of this statute and a genealogical search should give you a good start, not to mention keep you very busy. There are blood relatives somewhere. There are numerous companies that do genealogical searches. Just google them. You can also hire an estate attorney.