So the interview was successful but something else was found that led to her refusal. It could have been found before the interview and the interviewer did not know yet, after the interview, or while checking something she stated in the interview.
All the information is at https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/visa-denials.html
The embassy has found you materially misrepresented something on your application or during the process. NA212a6ci means she can never get a visa unless this is resolved. Your only recourse is for your sister or her lawyer (not a relative) to contact the embassy or consulate to seek clarification. No other recourse exists. The reason for denial, the process, and who to contact are all spelled out at the above link.