I know that its hard but its what you got into when you married him. His mother is not going to change so you will have to learn how to deal with it
just know that the changes you are making in your childrens diet is beneficial to them regardless of what anyone says...you are doing a good thing. as far as her showing your other children less love, I dont know how you deal with that because I wouldnt stand for it. remember that children sense things like that and it does affect them. That I would talk to her about asap. If you have to bring your husband into I would do that as well. good luck and Im sorry you are feeling this way.