Do you mean that this business wasn't mentioned in the Will? Generally beneficiaries of Wills can't act as witnesses or as scriveners (the drafters).
You need to take this Will to a reputable probate attorney ASAP for review.
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Your fact summary is not clear, but I assume that you meant to say that the business is not dealt with in the Will. This is not necessarily a problem, because Wills often do not mention specific assets.
What you need to determine is how the business interest is owned, and whether or not your father's share of the business will need to pass through probate. The lawyer should be a good source of information on this, and it would be quite natural for you to have questions, so the lawyer is the first person I would call.
I would ask:
1) How is the business owned?
2) What kind of business is it? Is it a corporation? An LLC? A partnership?
3) Is there a buy-sell agreement in place? What happens to your father's share as a result of his death?
That will get you started. If you are not comfortable with the answers, then you should immediately seek counsel from another lawyer. You will need to know the answers to determine whether probate is necessary. Who is named as personal representative/executor in the Will? That person is the one who would need to open an estate, if one is necessary.
There are many fine probate lawyers who are active avvo participants. You can click "find a lawyer" below and enter "probate" and your city, to find someone in your area.
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The will most likely says "all my real and personal property" shall be distributed as follows-
That would include any interest in his business interests.
The personal representative can chose whatever attorney they want to assist and advise.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.