Hello Harry
It sounds to me like you have already made your mind up. If you go you are going to drink, but you are going to swipe some booze and go drink alone?
I would avoid that situation like the plague. If you do go and drink you will have relapsed twice in a very short time, and it will be even harder to stop.
The way you are explaining it , you sound like you don't want to go.
I realize this is a family thing. But you say you lost your father to this. If they don't understand after that when will they?
The first rule of this is sobriety comes first. If you are going to stay sober it has to.
If you go to this thing and get the urge to drink, leaving in the middle of a family get together will be very awkward. It would be so much easier to drink than leave.
Of course I don't know you, so if you sit home you might also be just as tempted. Myself, it would be a lot easier at home with a couple good movies, and a pizza. Maybe call about the time you know your sister is 3 sheets to the wind and listen to how loco she sounds (or maybe not)
Just my off the wall opinion
Fred