There is no scientific research supporting the idea that particular combinations of food cause weight gain, or that other combinations promote weight loss. Your sister may lose weight on the diet, simply because she is eating less calories than she is used to.
The "secret" behind many fad diets is that they are actually very low in calories even though they may claim not to be low calorie diets. Complicated and restrictive diets generally lead to significantly less food being eaten and temporary weight loss. Unfortunately is difficult and unhealthy to sustain very low food intake over time. Many people can the weight back because the simply can't maintain their energy on such small amounts of food.
Following the Food Guide Pyramid and the RDAs for your age group is the best way to ensure adequate nutritional intake. If you are trying lose weight or to maintain a healthy weight you need to pay careful attention to portion size. Remember that all food can fit in a healthy diet and that eating a variety of foods at every meal is important. Variety helps provide the nutrients you need, and helps minimize hunger and low energy between meals. Try to include at least 3 different food groups at all meals. For instance, if you grab a whole grain bagel for a quick breakfast, add some cheese and a glass of orange juice. When you look at your plate, you should see a variety of colors and textures in your food choices.