The fluff about Banshees in WC3 was simple but efficient : Banshees are not mere puppets, but actively malevolent because they are jealous of the living. And obviously, the Windrunner sisters were very close. Sure people of bad faith will say that this is because Sylvanas and Vereesa are essentially expies of Aleria, but the three sisters took the same path, had the same occupation.
In War Crimes, Sylvanas problems with Vereesa are obvious-what about her problems with Aleria (in addition of the obvious, that they don't get along well). Aleria is alive, with a family, a loving husband,...