It appears from your description that your sister may meet the requirements of "qualifying child" for you. She needs to meet these requirements in order for you to claim her. I will post them underneath. The most important things to evaluate are whether or not she provided more than 1/2 of her support, and if she lived with you for more than 1/2 the year. Temporary absences to attend college do not count as time living apart if her primary residence is with you. Also, if you are able to claim her as a dependent, then you may use a tuition credit on your return, as payments made by the dependent are considered to be made by the person claiming the dependent for purposes of the Hope or Lifetime learning credit.
Here are the dependency rules, please let m e know if you need additional clarificiation:
Rules for all dependents include:
All dependents are either Qualifying Children or Qualifying Relatives.