Harry, first of all I sympathize with you greatly. It is terrible to hear that your father lost his life over this wicked substance. You are still here, however, and still have the incredible power of choice.
I know the anguish of sitting sober and miserable in the middle of a party. I can't express enough how happy I am that I've said NO over and over this year since I quit drinking in June. You have to remember that you are a special and unique creature of this planet and unfortunately, you don't get to enjoy the pleasure of drinking if you are indeed alcoholic. WORSE THINGS CAN HAPPEN! You know that!
Accept your condition, listen to all the people here trying to tell you it's going to be OK. You have to press on, or accept the same fate your father did. It is entirely up to you and in your hands. However I think if you persevere, even if it means going outside and screaming and punching things while everyone is inside drinking, you will thank yourself immensely later on. Go to a meeting before the party if you think that will help you. Tell your sister that you desperately need her to support your decision not to drink. Be an individual and make your own individual choices. You'll be amazed to find that down the road, people will begin to look up to you and ask you how you did it, as they see their own lives as possibly being improved by the cessation of drink.
Good luck to you. This is a hard time of the year for us booze hounds but nothing is impossible. You know the potential to do whatever you want lies only within you.