Wow this is a difficult situation. I would be honest about your intentions. Say that their daughter touched your heart. You might also want to say that you have no intentions of disrupting their family. And you may consider that they have not told her she is adopted (if she was adopted young) so that you may mention you would not disrespect any of their wishes in this regard. I think it would also be very appropriate to introduce yourself. In my culture it is important to introduce yourself well. Perhaps that is true for this family as well. They may want to know who you are, how you know their daughter, where you work, how old you are, if you have a good family life, etc.. Really any information you feel comfortable divulging. I would also consider consulting someone from the culture so that you may understand how to approach this letter from a culturally appropriate standpoint. If, of course, it was Chinese or another culture from your own, family who adopted this baby. You might also want to consider having it translated as well if that is the case and providing both an English and a translated copy of your letter. I really hope this works out for you.