This really depends on the actual documentation and the practice in your state. If by documentation you mean a promissory note, repayment should work. But if it’s simply documentation that the money came from your sister, it probably won’t. To be certain you would really have to consult with a local elder law attorney. The other option would be to reimburse your sister, making sure that she understands that if it doesn’t work she’ll have to return the money to your father.
Learn more about Medicaid’s asset transfer rules.
Learn more about promissory notes and Medicaid.