"小客人來自外省小鎮，是被收養的"

 

Do you mean "foster care"?

Keep the food simple. Ask about her diet at home, and try to duplicate.  17-year-olds don't have developed taste buds, especially if daily foods are plain.

Don't be distracted by her habits that are different from yours. If she is from foster care, these may not be taught at home.

Ask about her foster parents. How may kids are in the family. What the daily routines are. Try to discern the quality of her relationship with her foster parents.

She may be (unconsciously) trying to keep a distance by not making herself feel at home.  She realizes that you and your kids are a real family, which she doesn't have.  You need to be aware of the hurt she might feel inside (even though she is old enough to have gotten used to it.)

An opportunity for your own kids to learn that there are others who think and act differently, and a chance to find out what the guest is good at, by not being distracted by what is on the surface. 

A chance to get your own kids to appreciate what they have.

(You are no different from the many foster parents she has been placed under. So, don't expect her to open up to you; it is normal, she is just protecting herself.  Adjust your expectations accordingly.)