Do a bit of research on few family members with help from your fiance....eg the hobby of one auntie or what one cousin is studying etc. And perhaps even try to remember some of their names. It will be easier to talk of a topic when you have pre-thought about it. Also will impress them. This will in turn boost your confidence...and then it will be easier to go with the flow.
Good Luck...or wait...I guess I am too late?? Hope it went well.