Welcome. I'm Dr. Bruce and I've been a small animal veterinarian for over 15 years. Thank you for your question. That isn't surprising that Sister wants to eat the other foods - the Ceasar's and the chicken are definitely more flavorful. She knows what tastes better. The important thing here is to realize that you are making health / diet changes for her that will help her to lead a healthier and longer life. I'm assuming that your vet made this recommendation for the weight loss diet as she is needing to lose some weight. Being overweight puts more stress on the cardiovascular system, the joints, the respiratory system, and can lead to other health issues like diabetes
The Hill's r/d is as low of a calorie of diet as that company makes. In a situation like this, you could ask your vet to switch to Hill's w/d as this is a low calorie diet (not as low as r/d), but definitely good for weight loss and it does seem to be more palatable. I personally have my own cats on the feline version to help with their weight (I inherited a cat that was grossly overweight and with persistence this is being corrected).
I know you don't want her to not eat, but she's trying to train you to give her the foods she wants. You have to be tough and realize that in the long run you are doing what is best for her. In a way I can compare this to having kids. My own kids would love to eat cookies for breakfast, lunch and dinner. They give me sad eyes
and say why not when they ask for them, but I don't do that as I know they need good nutrition to be healthy. She won't starve to death
with her new diets as she will eat when the hunger is enough. You are again helping to train her to the new foods.