Legal Question in Wills and Trusts in North Carolina

You have kind of answered your own question. Your step-grandfather's will is controlling here. If he left everything to his wife (your grandmother) and she predeceased your step-grandfather, what, if anything does your step-grandfather's will say about what will happen to your grandmother's share?

Whether you get anything depends on the will. If it mentions you, then you are a beneficiary. If the will mentions beneficiaries other than you, then you get nothing. If the will says nothing, then a partial intestacy would result. In such case, the heirs of your step-grandfather, i.e., his children or grandchildren, or his adopted or blood relatives would inherit.

As he is only your step-grandfather, I assume that there was no explicit or equitable adoption of your father or you. Federal law is irrelevant; estates and will questions are purely a matter of state law.

Anyone can probate a will and serve as the personal representative. The personal representative's duties are to ascertain what the decedent (your step-grandfather) owned and what he owed, to pay any just debts/claims against the estate and to distribute what is left to the heirs. You could probate the will as long as you are otherwise qualified. However, unless you are a beneficiary, I am not sure why you would want the job.