Medicare will pay entirely for the nursing home for up to 20 days and partially for up to an additional 80, for a total of 100. Any coverage is dependent on your sister needing and receiving skilled care, which seems likely for at least part of this time. The facility should give your sister (or whoever is acting for her) a so-called Notice of Non-Coverage before the Medicare coverage is ended. If your sister has a Medigap policy or Medicaid coverage, it will cover the Medicare co-payment that begins on day 21.