K.W.
Before you can teach her how to eat healthy, you need to learn to accept her for who she is. Of course having a weight problem is not a good thing, and will cause other problems down the line, but most people overeat because of low self esteem or other issues going on, and if she has a beautiful thin stepmother who is CHAINING AND LOCKING UP THE FOOD (WHAT THE HELL???), that is going to make those issues worse and she will only end up eating more. Instead of focusing on her appearance, try getting to know who she is on the inside and loving her for that person. Once she feels she can trust you to care about her no matter what, she will listen to what you have to say about becoming healthy. Until then, she won't hear a word out of your mouth. I have struggled with my weight for years and have recently gone from a size 18 to a size 10 and it wasn't because someone humiliated me into it--I had to get things right within me before I could change myself without. You are only making a bad situation worse by what you're doing. Please, please try a different approach. Spend time with her. Talk to her. Most of all, LISTEN to her. But please stop humiliating her. Good luck.