I'm a young teenage boy (14 years old) and I'm writing a letter of thanks to my German exchange family for having me, and I'm having some problems with what I should use: Du, Sie or Ihr. I assume I call their 15 year old son by "du", as he is a child at informal levels. And I'm fairly sure I should refer to the parents, as "Sie", even though I call them by their first names (correct me if I'm wrong). However, the main problems are these two: Do I use Sie or Ihr when talking to both the parents? Do I use Du or Sie for the 20-year-old daughter of the parents?