With the child that asked for a sandwich--we don't know exactly what OP said to her, but even if the OP didn't explain things to the kid, a good guest still abides by their host's house's rules even if they aren't told the reason behind all of them. Explaining things would be a courtesy, and isn't required to still be a good host. But if the child is young enough, then I can see that she might not have known this. SIL's duty, when informed, is to explain this to the kid, as always when a child doesn't understand an etiquette situation. I give the child a free pass on asking OP for food, since she possibly just didn't know. Asking SIL also gets a free pass if the child is young enough not to have experienced this before. But SIL's reaction (not to teach the child the right thing to do) makes this a little hard to believe.
We don't know if the child went and told SIL to make OP get her some food (oh so rude!) or if the child merely went and asked SIL for something to eat directly, and SIL tried to turn it on the OP. SIL should have turned this into a teaching moment.
Kid: "SIL, can you get me a sandwich to eat? I asked OP, but she said no."
SIL: (should have said) "Ah, in that case, OP is the host, so you need to do as she says instead of asking me to do it anyway. That's what you do when you're a guest: follow the rules the host has."
SIL: (maybe actually said something like) "She didn't get you food when you wanted it like I always do? Oh no! How uncharitable of her! I'll go sort that out right away!"
SIL's way of feeding her children is certainly not my cup of tea. Not sitting down to eat and just eating whenever are not the kinds of things I'd be comfortable with. But I'd have no problem with it, though, if it weren't for the fact that she doesn't seem to be overly willing to teach her children that what is acceptable at home is not what is necessarily considered polite elsewhere, and how to eat in a more etiquette-approved way when they're guests.