Only werewolves up to the 4th generation (whether by being bitten or born into it) are supposed to be able to control when they turn. Since at the time Kate was turned all the monster alphas were dead (barring the alpha vampire), that would mean Kate has to be a 4th generation werewolf since she can control when she changes and the previous two werewolves in the pedigree line were shown (her friend who turned her and the professor who turned him), meaning when she turned her sister, her sister would be at least a 5th generation werewolf and unable to change at will like she was shown to when Sam and Dean confronted her.