Legal Question in Wills and Trusts in Pennsylvania

No. First, PA no longer recognizes common law marriages but since you indicate that the common law lmarriage occurred such a long time ago, we will assume for the purpose of your post that it was valid. In reality, just because people live together does not a valid common law marriage make. There are other requirements and the surviving putative common law spouse needs to visit a probate attorney and find out if this really met the requirements. Also, the requirements vary in state to state so you would need to know where the "marriage" actually occurred to determine its validity.

Common law marriage has nothing to do with the ownership of land. Whose name is on the deed? If the house is titled jointly in name of both people as either a tenancy-by-the-entireties (available only to a husband and wife) then the land automatically passes to the surviving spouse as of the date of death. Same result if the land was owned by spouse A and spouse B as a joint tenancy with right of survivorship.

If the deceased spouse solely owned the land, then there is a big problem. Even if a regular marriage existed this would still need to go through probate. If a valid marriage (common law or otherwise existed), the next question is whether there is a will. If so, the land passes as per the will to the named beneficiary. If there is no will, then the land passes to the heirs under the intestacy law which is the spouse and any biological or adopted children. If there are no children then the spouse is the sole beneficiary, but if there are two or more children the spouse only gets about 1/3rd of the property. However, if a valid marriage does not exist then the surviving "spouse" gets nothing.

I don't know what you mean by "taken over." We are not living in the wild west and while squatters may take over property in some places I don't think that is what you mean. Mortgages do not matter. Again, whose name is on the deed? That is who owns the land. If deceased spouse just started making payments on a delinquent mortgage that means nothing. The land would still be titled in that case in the name of the survivor and survivor owns the property. Survivor will have a problem though unless the survivor has money now because survivor will have to either sell the home or figure out a way to pay for it.

Survivor needs to see a probate attorney to figure out who owns what, whether an estate needs to be probated for deceased person (you do not indicate what assets deceased had because if there was no house and no other assets there may be no need for probate) and whether a valid common law marriage existed.