I've been reading many of these replies with interest and I can imagine alot of you would think that I feed my dd dreadfully.
I try to make her fresh home-made meals as often as possible (although been a bit lax recently due to morning sickness) and I make sure she eats fruit/veg every day, though we don't manage the 5 portions.
I must admit though I am not really that bothered if she has a few biscuits between meals in a day and at one point she was having ice cream as dessert after dinner most days (as it is her favourite). I have a fairly laid back attitude to food and yes dd does like the occasional McDonalds, although her favourite dinner is roast.
She is not overweight and her teeth are in good condition (mainly because she rarely drinks anything except water, her choice) and is generally a very healthy little girl. I must admit that when she was younger, I did restrict her diet alot more than I do now, but I no longer feel the need to do that.
My only taboos really are letting her have fizzy (she is allowed sips of mine on occasion or her own at parties) and I prefer her having oven chips etc as opposed to the take-away alternative. However, having read this thread fully, I seem to be pretty lax.
OP yanbu to feel a little concerned about your SIL's attitude to food as that is what will cause problems later on, I think she has set herself up for a very hard time. I am presuming when he is at school and invited to parties a bag of his own food will not go down well and he may be left uninvited in future, or he could well get picked on.
Also, as others said, having a healthy balanced view that ALL foods are good to eat (some frequently, others as a treat) is the best way to go. Food should be a pleasure, not a chore and complete restriction will only end up making the child unhappy.