I tried being soft and general with my little sister, didn't work at all. The point at which she stopped gaining and started tending towards human size was when my mother wanted to introduce me to some girls and I made a point of asking if they were fat, saying that I have no interest in meeting fat girls. She then told my little sister to take note of that and I followed up by saying that I was ashamed to be seen with [even fatter stepsister on other side] in public. Granted, there could have been other events around that time, but I don't think she realised just how repulsive obesity is until she was told it straight. Too early to say if it will stick, but she's gone from upper 95th percentile in BMI for her age to just over the midpoint for the healthy BMI range. Tough love and all that