im being punished for something i didnt do?

so recently a kid at my school who i've been having problems with for a long time
has been spreading rumors that i made a tried to fight him. eventually my sister found out about this. he went to her and told her that i threatened to beat him up. sh told me to say sorry and i told her i didnt do anything to him and she grounded me for two weeks i have video evidence on my phone that proves her wrong but she took that so i cant show it to her. eventually i gave in and apologized. i honestly didnt know what to do she kept yelling and screaming and interrupting me everyttime i tried to talk to her. i told her about the video and she went on my phone and deleted it, didnt even watch it. what should i do now?