If she was a full time student for at least part of 5 different months, then you would treat the time on the mission trip as a temporary absence and you would tell turbotax that she lived with you the whole year. That should qualify her as a dependent. If she is not a full time student, then she is too old to be a qualifying child dependent. She might be a qualifying relative dependent, which has different tests -- it doesn't matter where she lived, but you would need to pay more than half her living expenses for the whole year, and she has to earn less than $4050.
