You are an adult now. Therefore, extricate yourself from this situation. If you don't want to be sexual with a family member--- say BUZZZZZZ OFFFFF. And mean it ad he should leave you alone?
Do you have a way to record him? Could you tell your grandma and say you simply don't ever want to be alone with him? Maybe she could let you sleep in her room.
How old are you sweetie? I think you A. tell your grandparents. Whether they believe you fully or not, it puts your uncle on notice that you want it to stop and won't quietly take it. And B. tell someone at your school like a counselor.
This can't go on. How old are you and how old is he?