Legal Question in Wills and Trusts in Pennsylvania

This is called an "in terrorem" (Latin for "in terror") clause. I don't think these are necessarily a good idea as courts will not enforce them if the disgruntled heir had a good reason to challenge the will. A disgruntled heir cannot challenge a will for no reason; they need to have reasonable grounds (fraud, undue influence or lack of mental capacity) that would have to be proven by medical doctors or relatives with knowledge of your mother's mental condition. Also, litigation is very expensive and unless the disgruntled heir has money or is almost assured of getting an inheritance, then the heir will not be able to challenge the will as they will not be able to afford counsel.

It would be better for your mother to decide what she wants. Children do not have a right to inherit from their parents the same way that spouses do. Your parent can leave you nothing if they desire. If your mother wants to exclude a child, she needs to say something like "I have made no provision for my child, X, not out of lack of love and affection, but for my own personal reasons."

If she does want to include all of her children, then there will be no harm in adding an in terrorem clause. However, she should make sure that it is done through an attorney so that it is drafted properly. She also needs to realize that if valid grounds exist for challenging the will, then the courts will ignore the in terrorem clause.