The baby wipes thing is freaking me out too. I have chemical sensitivities, and there are about a thousand things that chemical ingredients can do to a person, short term (disturbances to the nervous system, dizziness, nausea, sleep and breathing disturbances, changes in cognitive function, etc.) and long term (hormone disruption, organ poisoning, cancers, etc.). There's a very informative website that reviews different brands of all kinds of toiletries and personal products called Skin Deep Safety (http://www.cosmeticsdatabase.com/)
Baby wipes and many other personal care products are not regulated for safety. There are 87 brands of wipes listed on this site, using different ingredients. Some are pretty innocuous WHEN USED ON THE SKIN, and some are considered fairly hazardous. None of them are rated for ingestion by mouth, and that would increase the potential hazards posed by many of the ingredients. see http://www.cosmeticsdatabase.com/browse.php?category=baby... for more detail. (This is the nastiest end of the list – you can page backward to the less dangerous brands.)
So your niece could be becoming slowly poisoned. And the ingredients in many of these wipes can cause "neurotoxicity" and "biochemical or cellular level changes, "organ toxicity" and other effects that could influence her health in any number of ways, including increasing or decreasing her appetite for certain foods or making her nervous system too sensitive to certain kinds of input (like food textures). And food allergies themselves, as a milk allergy, for example, can sometimes be expressed as a craving for the very thing one is allergic to. And cow's milk is not a complete food for humans. Even as the body begins to suffer from unbalanced nutrition, tolerance builds up to the body's reactions, but this results in a "crash" of sorts when the substance is withdrawn, and that crash can be distressing and uncomfortable.
In short, too much milk could be making your niece unable to tolerate having the milk taken away, and baby wipe ingredients could have any number of other health effects. Depending on the brand, she could be ingesting truly dangerous levels of toxic ingredients. Click through the list above to find the brand her parents are giving her, then click on that brand to find out what it contains and what the possible effects of those ingredients are. This website is a fantastic resource for quickly assessing all sorts of personal products we bring into our lives.
Be alarmed, and let your alarm show. Please don't let your niece's parents go on ignoring the possible harm these habits can cause her, including poisoning and malnutrition. Even if her main problem is sensory dysfunction, she needs help.