It's my younger sister, but the reason shes gaining weight is that her thyroid gland is not producing enough thyroxine, so she needs the artificial hormone in pill form. I assume your sister is on thyroxine, but I'm not a medical expert, it's just the condition runs in our family and she eats a low calorie diet, and she is the only overweight member of our family. I'm just pissed that fat people lie and use a thyroid problem as an excuse when they don't have it, they do the same with pcos, it hurts the people who actually have these things.