Legal Question in Wills and Trusts in Pennsylvania

Your son needs to consult an estate planning attorney as what you propose is NOT a good idea.

If your son has his own son who is under age 18, then your son needs to make a will (or trust) making his minor child the beneficiary of his assets. This would of course include any real property owned by your son. If the child is under age, your son can create a trust within the will or revocable living trust and provide that his brother can manage the property and other assets for the benefit of the child and that when the child reaches a certain age, he can fully receive the assets free of any trust. Your son is going to have to find a way to pay for the mortgage after his death, by either providing that the home is rented (thus bringing in rental income) or obtaining a life insurance policy and directing that the trust use the proceeds to satisfy the mortgage.

Your son can do this regardless of whether or not he marries the girlfriend. If in fact he does get married to the girlfriend or someone else, then your son can simply amend the will or trust and make provision for his wife and for his son and any other children born to or adopted by your son.

If your son dies and is not married to the girlfriend, she and her children get nothing from your son's estate (the only exception is where your son adopted these children by other men) or where your son makes a will that specifically includes the girlfriend and her children.

If your son is married to the girl and then passes away without making a will or trust, the girlfriend and your son's biological or adopted children would split any assets, including the house. The girlfriend's children by other men would not inherit by virtue of her marriage unless they were either adopted by your son or named in a will or trust.

Of course, if your son and his girlfriend buy any real property together and own it either as a joint tenancy with right of survivorship (if not married) or as husband and wife (tenancy by the entireties), or if the girlfriend is named as a beneficiary on any retirement, pension or life insurance, then such assets would go to the girflriend as they are "beneficiary designated" assets and would not be a part of probate of any will or administration of a trust.

The best thing that you could do for your son is strongly encourage him that if he has a minor child then he really really needs to sit down with an estate planning attorney and have a will or trust drafted. If he gets married, divorced or goes through any other life-changing event then he needs to revisit his will/trust and make changes if necessary.