The Sackville-Baggs died from a chlorea epidemic.
Chlorea is a disease found in infected drinking water. Because the disease spread throughout their vast clan and possibly village, there were not enough people to bury all of them. Without a burial, the deceased become vampires.
Frederick's actually Romanian.
He looks more like Dracula,( maybe he's Dracula cousin )than a Scott. Once he moved to Scotland, he changed his name and married Frieda.