can i deduct the student loan interest i paid for my sibling? I am cosigner on the loan and she can not pay back the loan

No.  You probably don't meet rule #3.

You can claim the interest deduction, if you meet the three requirements:

1. You paid the interest
2. You are legally obligated to pay it (co-signing counts)
3. The student was your dependent, at the time the loan was used to pay for qualified educational expenses

It depends. Is your sibling your dependent? The cosigner of a student loan can claim the student loan interest deduction on their tax return. To claim the student loan interest deduction, the borrower must be legally obligated to make the payments on the student loans and the student must have been  a dependent of the cosigner when the loan was taken. Cosigning a loan counts as a legal obligation to make the payments.

Please review the IRS Topic # 45 Student Loan Interest Deduction for more information.

Refer to the TurboTax article Where do I enter student loan interest (Form 1098-E)? for instructions.