dont kick her ass, cause then it will be an isolated incident. what you need to do is call her out explicitly over facebook, so all her friends can see it. get your friends to comment on the picture too, saying stuff like "thats not you, thats (your name here)" and "youre a phony you little shit"
that way, all of her friends, when they log into facebook, will see right on their homepage, "32925 people commented on (your sister's name's picture)"
and then theyll be like, yo, wass da big ruckus? then theyll be like. OMGZ i just lost so much respeck for you
pain is temporary. this isnt