Witnesses, Eligibility
Generally, any person who is at least eighteen years of age may be a subscribing witness. 

Pennsylvania does not penalize subscribing witnesses who are also beneficiaries of the will.  However, any person who has also signed the will for the testator may not also sign as a witness.  Any person who has subscribed the testator's mark is also prohibited from serving as a witness.

See: Witnesses, Generally; Signing By Mark; Signing By Another Person; Witnesses, Advantages