It does happen though it is rare. I think it's happened to me about 4 or 5 times playing the game since it was first released. The trick is to keep taxes low. None will join your faction if your taxes are too high. This may also be related the happiness of the province. The maximum is 200% but the game can probably handle far in excess of that internally, it just displays 200% to the user though. It seems that factions are more likely to join your faction when the happiness is ridiculously high. I had this happen as the English years ago, when I inherited the Danes' lands. Basically I married my heirs to all of his daughters, kept takes on normal, built town watches, watch towers, border forts churches etc over , to improve happiness and then assassinated the Danish family line. Bang all of their provinces joined my faction. I've tried this a few times ever since with various faction and have had limited success, but it does happen occasionally.