Is doing the opposite of what I want a teenage expression of anger or is it something that she has decided gets favorable response from watching her sister do it?  If it is just a teenage thing - whatever - I can deal with this just like I dealt with toddler-hood.  If she has seen her sister get a favorable response - when?  and what can I do about it?  It is definitely one of the more annoying characteristics of my girls when they are angry.