maybe her magic just has a continent-wide range? one direct link can leads to a linkage of all other people. Because I dont think even she can physically travel the entire place on her own.
NoNick: I don't know about the Dimaria and Brandish relationship thing though... She has always been sort of sadistic person from what I've seen though. it might be just a combination of linkage. like when Meredy had hatred against Grey, she managed to use Juvia's link to link herself into it (still has the love symbol). So it might've channeled her hatred XD