There're certain "rules" or as to who is supposed to "offer" the "du". (The "du" s usually something that is offered - not asked for.) If you're in a hierarchy, then obviously the superior is the one to offer it first. Otherwise you normally go by age, which means the older one is supposed to make the first step. There are certain occasions where everyone uses "du". Some people say above 2000m of altitude there's no "Sie", I've hardly heard a "Sie" in the sauna (where everyone is naked and thus - in a way - considered equal).
Since those things don't apply in your case, there's nothing wrong with using "Sie" in the first place and I'd expect you to be offered the "du" quite soon. It's not unlikely to be the first thing that's done once you meet in person...
Just a few more remarks:
Germans usually know that this is a difficult issue for foreigners and I think it's close to impossible for you host family to feel offended no matter what you do.
As for the "rules", they vary widely from individual to individual. Older people usually prefer "Sie" where as younger people tend to care much less. I'm almost 40 and I don't mind the "du" except for very special occasions such as a policeman giving me a ticket. On the internet I normally default to "du" (unless I know the people personally) whereas in real life I normally use "Sie" for adults.
When I was young, even small children were taught to use "Sie" for adults. Now my wife (elementary school teacher) starts teaching that somewhere around 2nd grade.
Using "Sie" normally implies using the last name (Herr/Frau ). (The old "Fräulein" for "Miss" is deprecated - perhaps because the marital status is considered a private issue.) There are some exceptions for nurses, monks and nuns who are likely be called by their first name but still addressed with "Sie".
Christians at a specific occasion in a service use "du" for their pastor or priest and even the Pope (though in Latin), but only then and not in a normal conversation.
As far as writing goes, "Sie" is always capitalized whereas "du" is usually not (any more). The capitalization is meant to express a special respect. In the internet, some consider an uncapitalized "sie" as an insult. I tend to disobey the rule for "du" which I capitalize if I appreciate the conversation, but this is just a personal habit.
To add a bit more "useless knowledge" as my wife would call it:
Using "du" for a policeman is likely considered an insult with you can actually be fined for (not likely for a foreigner, I guess) unless you are Dieter Bohlen who officially gets away with it. He argued that he's using "du" for absolutely everyone and because of that in his case it's not an insult. Don't know if this is true, but I once read that he is supposed to have said that he even uses "du" to address God (which everyone does) and there's no higher authority than Him.