One of our dogs rarely ever get snacks. He hates and will not touch human food. He does get a few dog biscuits maybe once a week.
Our other dog is in service dog training training, so he gets treats almost day as training tools. As a service dog trainee, Mario never gets human food. He has strict rules he must obey and he must behave a specific way. Yet, he is the happiest dog I know and the best trained in our entire family. We went to a party last night, and 8 adults postponed a game they were playing for 30 minutes simply to watch the playful antics of Mario engaging the four other dogs there into playing with him, even the one that did not want to play ended up playing.
HOWEVER, you need work on becoming secure in your choices and ignore what everyone else does. Your dogs are not comparing notes with other dogs and they are not feeling resentful about treats. If they are healthy and happy, you should be happy. If your routine works for you, then good for you and better for them.
As for your MIL, she has created the environment she can live with which includes a picky eating dog. Likely her feeding habits have a huge contribution to that situation. If you do not want what she has feeding environment wise, and are happy with the environment you already have, sit back and enjoy what you have.
There is no reason to stress over this. Let what she says roll off since you are confident (work on this
) with the choices you have made and the outcome those choices have lead to which include happy, healthy, well adjusted dogs