So there has been a bit of an issue lately at our local primary (the one my kids went to, they're teens now but it's a small community, so we still know a lot of people there) with people bringing additional kids (siblings) to parties and it's perceived as quite cheeky, especially when they're left and hosting parents expected to supervise and feed kids they didn't invite.
There was a whole-class party at a leisure centre last weekend at which the hosting mum had done little lunch boxes for each child with the usual - rolls, fairy cakes, fruit, veg sticks, crisps etc. There were unfortunately several uninvited siblings dumped by their parents at the start of the party.
If I were the hosting parent and I knew this wa likely to happen, I would have either put a note on the invitation that I was catering for the children individually, so please do NOT bring additional children, or brought a few extra boxes of food - I'd be pissed off at the cheekiness, but I wouldn't see a child go hungry.
This mum had brought a little bin with a sign on it saying "Yuck Bin" with a vomiting emoji 
which she passed around for all the children to put the items they didn't like from their food boxes. Then the uninvited siblings were allowed to take food from the Yuck Bin.
I wasn't there, obviously, but I have heard via friends that one of the mothers of the additional children flipped out at the end of the party when she was told by her child about how he was fed. There was a bit of a scene and the birthday child was upset. I can see her point tbh - she's a rude cow for dumping her children wholesale without asking first, but the hosting mum's way of dealing with it was horrible. AIBU to think that nobody has behaved very well here?