If I am not in the Will why should i be required to sign an Ackowledgment of Service and Assent to Probate Instanter?

You are being asked to sign the Acknowledgement and Assent because you are your father's heir (the only heir, based on the facts you stated in your answer). Without notifying the heir and either getting your consent to having the Will admitted to probate or having you notified by the court and then having you fail to respond in a timely manner, the Will can't be admitted to probate in solemn form. DO NOT sign the Acknowledgement. In it, you state that you have been provided with a copy of the Will. If you have not been given a copy of the Will, therefore, you can't even actually sign the Acknowledgement. Demand to see a complete copy of the Will before you sign anything. Better yet, find yourself an attorney that has probate and estate litigation experience and have the attorney demand the correct information, including a copy of the Will. Your father's girlfriend may be trying to hide something, or she may just not be getting good advice about how she needs to proceed. But you have rights and an interest in the estate as your father's heir, and you are entitled to protect them. Best wishes to you.