It sounds like she has reached a difficult stage in her life. It can be a bit odd to grow up,change from the child in the family and begin a new one as an adult, she may have aggreed with you about her brother but felt strong loyalty to him,feelings would have been divided for the first time. Another thought,has she been harbouring thoughts of uni for some time? maybe settling down in a home she is responsible for made her feel opportunity has passed by.Another thing about your own home is money, up to now would she have bought new clothes, been out a few nights a week and can no longer afford to. This may not be about 1 thing, just lots of new things came at her at once. She could just need time to get her head round it, there is also you in all this and your feelings so I would set a time limit. Christmas is less than a month, do you both have plans? how about contacing her saying you care enough to give her space to think but would like to know a definate answer by 15th dec so you know where to spend xmas.