Who does MIlitary Healthcare cover?

Military health care would not cover your sister unless she was living with you and was considered your dependent as an "exceptional family member." She would have to suffer some type of significant medical affliction that would require regular or constant medical care (e.g., autism, cerebral palsy, Down's Syndrome, muscular dystrophy, paralysis, etc,). So if your sister is generally a healthy woman, there's no way you could get military health care to cover her.