claiming a dependent

It depends. You may be able to claim your sibling if they qualify as a qualifying child or a qualifying relative. 

It appears she meets the age and relationship test  to be claimed as a qualifying child as she is under 19 and your sister:

• She must also have lived with you for more than 6 months during the tax year, and you provided more than half of her support. 
• She must be a U.S. citizen, resident alien, U.S. national, or a resident of Canada or Mexico. 
• If you were deployed on active duty while still providing her support, this counts toward the required number of months she lived with you. 
• You will need to retain proof of the relationship (e.g., birth certificates showing shared parents) and records illustrating where she lived and how much money you spent to provide support. Also, keep a copy of your deployment orders.

For more detailed information, you can refer to the following resources:

• TurboTax Help Article on Claiming Dependents
• IRS Rules for Claiming Dependents