My friend is spreading false rumors about my brother?

This girl is accusing my brother of sexual harassment and he didn't do anything to her. She is spreading false rumors about him that are not true, she was one of my closest friend and now she is doing this to my family. The date when this happened, my mom was with her, she said she was with her all day and that never happened. He's my brother and he will never do that
I can't stop crying seeing my mom and my brother cry . It just broke my heart and she is getting away with all of these and plus this guy I like is in love with her, he is my friend from along time and he just met her and he believes everything she says and plus she dated my little brother while liking my older one ( the one she's accusing). What should i do ? PLEASE PLEASE PLEASE HELP ME