How your sister is feeding him makes no sense to me. She's afraid of mad cow disease in fish? And yet she'll feed him animal products like cheese (from a cow) and eggs...which carry a salmonella risk, but she won't feed him jelly or ice cream. Ummm...
She, in my opinion, desperately needs to research basic nutrition needs. She is completely ignorant of any nutritional education and is therefore denying her son any nutritional value. She is also putting him at an extremely high risk for obesity, which in turn comes with all sorts of health issues.
Cheese is high in sodium and can create high blood pressure--yes, even in children--especially if the other main part of his diet is salty chips. Eggs are high in cholesterol, and I doubt she is only feeding him egg whites. So while I'm sure he's not lacking in protein and calcium...he could be lacking in iron and omega-3s, which is an essential fatty acid for brain development, found in fish and avocados.
If he's not taking a multivitamin, he is at high risk for vitamin and mineral deficiencies as well.
Since she's so concerned about meats being contaminated, she may want to consider buying organic meats.
But if she is set on maintaining a vegetarian diet for him, then she NEEDS to better educate herself about nutrition before she unintentionally feeds him long-term health issues.
There are lots of other sources of protein , what about beans and legumes maybe suggest to her that at least she gets more healthy food going, how about fish ,as she is his mom unless you can talk her into giving him a more balanced diet not certain how much you can help him,there are books out there and she could do some research about mad cow .Good Luck its great you are trying to help, maybe a heart to heart about your concern, how about suggesting a good Multi Vit with Vit C and B Complex
She feels its for his best as meat could give him CJD/mad cows disease?!?!?! She wont let him eat jelly, as in jelly & ice cream because similarly jelly contains gelatene & therefore animal product, exposing him to the same risks. When I look at him, he alwasys seems in a bit of a daze & doesnt fully focus on me. He seems to be behind, compared to my 2 year old daughter & Im trying to convince myself that this could just be them developing at different rates, but its always niggling away at me that it could be related to his poor (in my eyes) diet.
I need to understand this your sister eats meat but wont allow her child to have any protein ? I personally dont think what you have described here is a healthy diet ,what reason does she give for not giving the child it?