Giving zakaat to someone without informing him that it is zakaat

My Brother in Law (husband of My sister) has Died. My Sister has 3 Children. As a brother I am helping my sister more and more in financial matters. Also she is receiving some money from the company where my brother in law had worked. But some times this money is not enough. I tried to give my Zakat to my sister but she refused to accept it and said I will never accept zakat or Sadqa. In this situation can I give zakat to my sister without telling her or if she asked can I hide or tell a lie that I am giving her zakat.