RDAP is the only way to reduce a federal sentence. If she has not filed her paperwork beforehand to prepare for the qualifications of this program it is doubtful she can get in. The reduction for RDAP inmates is 12 months; 9 months for sentences of 24 months or less.
Federal inmates have to do 85% of their sentence which is 15.3 months in your sister's case.
https://www.inmateaid.com/ask-the-inmate/my-sister-got-sentence-to-18-months-in-fpc-bryan-texas-is-there-a-way-they-can-reduce-her-sentence-and-what-does-she-need-to-do#answer
Accepted Answer Date Created: August 13,2015