What should I do? Will she really do this to me?

Okay,me and my best guy friend are really close. We just started Highschool and we DO NOT like each other. We think of each other as brother and sister. but everyone thinks that we're dating and like each other. Then people started to bother me asking if I like my friend but I said no everytime until one time I just had enough and told them yes but in truth I really don't. My friend, he knows the truth. But then my other friend asked me the same thing. She kept pestering me until i said yes but afterwards I said i was joking. She then told everyone in our grade and they all know. I kept telling her it was a joke but then she thinks I'm lying. She said she's going to tell everyone including my family that "I'm getting a boyfriend and that I'm lying to my mother" My mother knows the situation. What am I suppose to do know?