You absolutely need an attorney, at a minimum, a local one to consult with. Unfortunately I am on the clear opposite end of the state from you hear in Delaware County. If your father died without a will there is a set of laws called the laws of intestacy which will divide up an estate of a person who passed away intestate (i.e. without a will). Do not sign a thing until you retain counsel.
Good luck. Contact my offices via the phone if we can be of any further assistance.
You and your siblings as children of a prior relationship (not of his widow) should be entitled to a share of your father's estate under PA law; your stepmother is apparently attempting to push you into giving up your property interests. You should contact a local attorney to protect your interests; you might also consider petitioning the court to be named estate administrator. If your stepmother has already filed papers for estate administration, you should be able to find out by contacting the Erie County Register of Wills (that office also administers estates without wills), and if she's filed for appointment, you have the right to challenge it with the court.
Of course, as with all of my online answers, my advice is limited by the brevity of your question and the facts provided. Additional information would be required to provide definitive legal advice, so this answer isn't intended to, and does not, create an attorney-client relationship.