Praise be to Allah.
This girl has to tell her mother and her brothers so that they can put a stop to this crime before it happens. The girl must not sit with him in the place where he is sitting, rather she must keep away from him even if he is her mahram. The mahram – even if he is a father or brother – if there is no guarantee that he can be trusted (Allaah forbid), then it is not permissible to be near him, rather the guardian should prevent that from happening. If that can only be achieved by punishing or imprisoning him, then so be it. Whatever the case, this girl has to take every precaution to prevent this evil from happening.