You say that her sister is a permanent resident, but is SHE?1. If she is NOT a permanent resident, then the same rules as other helpers apply. Imm D will no longer approve "live out" helpers. There are some legal "live out" helpers, but they were approved many years ago (and continued to renew).
2. If she is a permanent resident, then of course you can hire her and she can live at her sister's house. But is she willing to accept a pay of $3940? She can earn much more as a waitress/sales assistant!