Each and every brother has a responsibility after the father to look after his sisters, regardless of what age group she is. Allah has made the brother a muhrim towards his sister, until she is married to her husband. 

Whenever the brother enters the sister's room it is important to make sure that brother knocks, no matter if the sister is younger then the brother. 

It is important to watch her lifestyle also, and to make sure it is according to Islam, and do not forget that one girl's responsibilities goes towards the father, brother, and husband. And if they are not caring of her Islamic values, then she would take them to hell fire.

If the father or husband dies, the full responsibility falls on the brother, not to leave his sister alone, until she finds the husband again. No one can give any excuse of his expenses towards his wife and children, and to ignore the sister.