I don't know, positive reinforcement might be ok. I guess it depends on what kind of attitude all three bring to it. I personally know that negative reinforcement is not the way to go. When I was in 4th grade I moved in with my father and his wife and was immediately accused of being a fat kid. from 4th grade to about sophomore year of high school, I was punished (when I say punished, I mean no TV, friends, basically and thing but books for months at a time. Not to mention the occasional beating) for gaining weight. Needless to say, puberty was not the most pleasant time of my life. I am now 24 and basically neurotic about my weight. My weights fluctuates like ocean tides. So take it from me, this positive reinforcement is better than the alternative.

p.s. have your parents teach her to eat smart, and if your family doesn't at meal times you sister should not be expected to eat smart at all. Everyone has to support her.