There is no point in hiding such a thing, you became a victim of male rape and now the person is threatening you for your sister which is worse than anything else. Of course, you must talk to your parents about this, you do not have to be afraid about anything. Make sure your sister is protected and safe, do not let her go out alone until this case is solved, and do not ignore the threat. Ask your parents to file a complaint against the boy and involve his family in this matter. You should raise your voice against any torture to you or anyone else.