without any knowledge of your sister's job, whether she's actually sick or not, or why you feel it's important she doesn't go to work, I don't think this is answerable. I will say that in the places I've worked and consulted, as well as the business I've run, the only time I would be ok with someone's sister calling in sick on their behalf would be if they were physically incapable of calling, texting, or emailing me themselves. Unconscious in the hospital level of sick. — Kate Gregory 2 mins ago