To Vous or Not to Vous

... that is the question.  I am sitting here at my work table in Vancouver looking at my enormous list of things to do in preparation for my return to Paris (e.g. apartment hunting, dossier preparation, travel plans, etc.) and somewhere in the Not Priority But Could Be Problematic section is scribbled "French family etiquette - research". 

The key question that I have (and which has been swimming at the back of my mind for the past few nights) is, when meeting the Partner's French Family, when do you use vous and when do you use tu?  It seems to me from my limited investigations thus far that the Parental & Grandparental Units always fall into the vous category, until you somehow mysteriously receive the Golden Invitation Into the Land of Tu.  But when is this invitation given?  How do I know I am receiving said invitation?

And what about siblings and extended family?  I've always assumed that I could tu-toyer the siblings around my age (I hope that was correct) but what about the older siblings?  (For the sake of this investigation's accuracy, the older siblings are about 10 years older than me.)  And I've been reading mixed things about aunts and uncles; some vous-voyer them, others tu-toyer.  Really.  Sometimes this language can be as inconsistent as onion soup.  (In Spanish, it's much easier.  When meeting The Family, you basically use usted with everyone who is significantly older than you.)  And if you can tu-toyer the aunts and uncles, what about great-aunts and great-uncles?  (I've already met one of the great-grand-uncles last summer, and since he was about the oldest person I have ever had a meal with, I basically vous-voye'd him til kingdom come.)

And lastly but most importantly... how do you navigate all of this whilst coming off as the beautiful, highly intelligent, charming, witty, loving, successful, and obviously very culturally aware international girlfriend that they simply cannot believe their very own son had the luck to snag?  :)