The IRS has established some relationship tests for taxpayers who wish to claim a deduction for qualifying relatives. In order to meet the relationship this test, the IRS requires that the dependent must either be related to the taxpayer is one of the following ways, or live with the taxpayer for an entire year, and the relationship must not violate local laws (Residency Test).
It would appear that in that in your circumstances where your Sister may perhaps have not lived with you the entire year your Sister would have failed the Residency Test and you would be precluded from claiming her as a dependent.
But, fortunately for you and many others in your situation, the IRS has created an exception for certain qualifying relationships who would be exempt from meeting the residency requirement. The IRS has stated that the following relatives would not be required to meet the Residency Test if the dependent will meet the relationship test for being claimed as a qualifying relative and if the dependent is related to the taxpayer in one of the following ways:
1. son or daughter, grandson or granddaughter, great grandson or great granddaughter, stepson or stepdaughter, or adopted child,
2. brother or sister, (You would qualify for a deduction under this provision)
3. half-brother or half-sister,
4. step-brother or step-sister,
5. mother or father, grandparent, great-grandparent,
6. stepmother or stepfather,
7. nephew or niece,
8. aunt or uncle,
9. son-in-law, daughter-in-law, brother-in-law, sister-in-law, father-in-law, or mother-in-law, or
10. foster child who was placed in your custody by court order or by an authorized government agency.