My sister does not believe in Santa Claus?

My little sister Kayla she is 5 years old and in September she started Kindergarten. Her friend Hannah from school told her that her older brother said Santa Claus is not real. My sister was very upset by this and ever since Tuesday (when Hannah said that) she hasnt been her usual goofy self! Shes usually running and dancing around singing and laughing and you cant do anything without her loud voice in the backround or her clinging to your leg laughing and talking but shes just walking round the house all silent and sometimes she comes crying to me! We arnt telling her the truth after all she is 5 but any ideas to cheer her up and get her believing again?