You do not give enough detail to answer the question as to the amount of time. More importantly, your sister should talk to her federal criminal defense lawyer. She would have been represented by a private lawyer or the federal public defender.
Sometimes you can avoid additional time if you get into a drug program after one slip. Here, unfortunately, she has three dirty tests. Still, her lawyer would want to explore the possibility of a program rather than straight custody for a violation of supervised release.
Tough situation. Hope she works it out after talking to her lawyer. Good luck!
One would need to know the offense of conviction to answer your question more precisely. The term of imprisonment that may be imposed upon revocation of supervised release is limited by statute to not more than five years for persons convicted of Class A felonies, except for certain Title 21 drug offenses; not more than three years for Class B felonies; not more than two years for Class C or D felonies; and not more than one year for Class E felonies. 18 U.S.C. § 3583(e)(3).
The policy now is that the fourth positive drug test ends with a motion to revoke supervised release. This is federal supervised release - she does not have "finish" any sentence. Supervised release is imposed without suspended time and the violation of its terms can result in an additional to the original sentence for violation - not reactivation of a suspended sentence. The federal system does not imposed a sentence of probation except for low level offenses. Your sister may have a had a sentence of probation, but probably not. Your sister probably still has a Federal Defender or an attorney from the CJA Panel. That's the person who will know the specific answers.