Steve - for the first drink you are offered, get yourself something very long, fizzy and non-alcoholic, especially if the weather's hot so that you don't start off the occasion with alcohol. As for seeming a failure when you have to talk to relatives, what is it about your life that makes you feel insecure? In the longer term, if this is an issue, resolve to do something about it, but for tomorrow, just think of a few current topics that you can comfortably talk about to anybody - the weather, what they are doing about holidays, any interesting places they have been recently, etc. Once you start chatting, it's surprising how one subject painlessly leads to another.
And shy are you worry you will get interested in any women of your own age? Are you already attached and fear being unfaithful? If not, just relax and enjoy their company. It will probably only be for a few hours unless something interesting develops.