Re: Du, Sie und Ihr
You can either just ask them how you should address them, as suggested by Severin, or you use Wroclaw's excellent advice:
It is a good way of avoiding choosing either "du" or "Sie", native speakers too use it. In some regions you even can use this half-informal "ihr" to address a single person; in Austria we do so on occasion if we are not sure wether to use "du" or Sie" - but in your case I would advice against that (as I don't know your guest parents, naturally ), only use "ihr" when addressing at least both parents.
Originally Posted by Wroclaw
"An esoteric may claim more nonsense in 5 minutes than a scientist may be able to disprove in his entire life." Vince Ebert, about fighting sciolism.