i have a 165 lb male rottweiller,so i deal w/agression daily. not from him ,but .....ill try to explain. if a person,anticipates that theres a prob. their pet, esp. dogs zone in on that uncomfortablevibe that you omit about a prob. arrising, so being the protective creatures they are,they do just what you describes as your problem.
try not to make a issue when your sis and the dogs are around each other. be w/in a safe grab of her, just in case,but try doing something to distract the dogs from your sis.....that strategy works for me.
good luck