Welcome to Yorkie Talk.  You are doing a very good thing to help this dog lose weight.  Try to talk to your mate's parents about the health implications.  All of the extra weight is not good for the legs, the trachea, and vital organs like heart and lungs.     
Do you have a photo to share?  What is her name?   
You can add enticements to her dry food that are low in fat and calories.  Green beans and steamed plain chicken breast.  Chicken will also boost metabolism.     
I would hold all of the treats, including the yogurt.  How was she eating 1/4 cup of yogurt twice a day when she was so small?  My big boys, 9+ pounds, can't do that.     
				__________________  Kristin, Max and Teddy          |