I'm still not a PA lawyer, but I don't think the designation of "marital property" has any effect on the analysis.  Marital property is a concept of domestic law and is applicable to the equitable division of property in a divorce. Since divorce is not possible after husband dies, there is no longer any marital property.

I am not aware of any law that would give heirs of a decedent the right to claim an interest in marital property a decedent may have had any rights to prior to death.   Marital property rights protect spouses, not heirs, in my opinion.

Are there any PA statutes, particularly probate or real property statutes, that refer to marital property in this context? 

Ponder these questions - could the decedent have deeded his marital property rights to his children prior to his death?  Would he have had any right to deed any interest in the wife's property to anyone prior to his death?  If not, then why would his heirs have any right in the property now?