Sounds like your DiL has issues which are not just cultural. I don’t think you can blame her fixation on milk on her culture. Traditionally, Chinese diet does not have much in the way of dairy products and formula milk has not been used ( maybe that’s why the manufacturers are seeing a potential huge market). Chinese babies would have rice with maybe mashed vegetables or bean curd, a little fish etc. Proper Chinese cooking has lots of vegetables, beans and rice, noodles, bean curd and less meat and fish than in the western diet. So maybe she thinks it’s the Western way to have lots of milk. Sounds like a bit of education needed. Maybe there is a Chinese cultural centre if you are in a city with any sort of Chinese population, where she could get cooking advice? Other than that, she should definitely be seen by the health visitor.