The Big sisters are one of the hardest enemies on Bioshock 2 but with a simple strategy you can kill them easily.  First use the winter blast, this will freeze them making it impossible for them to hurt you.  Then you can use any weapon to kill them, I prefer the machine gun as it is very powerful and you are at a safe area when they thaw.  When they do thaw simply freeze them again.  Using this strategy You can kill a big sister and take very little damage.  You can do the same with Big Daddies but when you do stop freezing them when they are about to die so you can loot them.  Do not worry about this while fighting a Big Sister as you can loot them even if you kill them while they are frozen.