I would tell them that she is overweight and her health is at risk and the vet said no more table food. Make sure they know how serious this is. Of course if they are like most in-laws they will disreguard everything you say...

. I think that she is overweight because of the table food and snacks. That is why she doesn't eat much of her kibble. If you stop giving the kibble and homecooking then I would feed twice a day and that is it. Good luck.