I was going to leave, but I will reply to you Floggingmolly.
Re "What does it matter where I'm from, Italian?!"
It doesn't
It was a turn of phrase, like, 'it's not like that round here', but really it is irrelevant so sorry no it does not matter where you are from. (I am from the countryside, just for the record.)
Re "Why would anybody prepare enough food / party bags to ensure several siblings per invited guest (potentially) can be fed if they happen to roll up?"
It is my experience that siblings to a party would not at all expect a party bag. It is also my experience, having run 14 children's parties myself and attended too many to count, that there is always a lot of food and plenty for all. Parents are usually invited to eat last and many don't. The exception is when it is party boxes at a place like a soft play place and then i would not expect anyone to bring a sibling and expect to be fed,
Re "It's simply common courtesy to let the hostess know in advance how many to cater for." Of course it is and i always would do so if i ever had to take my child.
My kids have a big age gap, almost 6 years and I've got a husband who doesn't work weekends so there is no need ever to bring an uninvited sibling to a party. But my friends include single parents and those in the army so there are parents who cannot get child care because they are parenting alone or their partner works unsociable hours.
I guess I am just wanting to point out that it does happen, people should warn in advance, but even so sometimes people can get let down on the day at last minute and bring a sib. That is normal in my circle of friends but only for parties at home and village hall/sports centre.
It does lesson off as children get older because you do not need to stay with them at the party.
Re "You seem to think advance warning is unnecessary." I think advance warning is polite, I think it is best, I think it is what I would do every single time
But in the grand scheme of things I would much rather a child attended, than did not attend, and much rather a parent stayed, than did not stay, even if this meant arriving unannounced with an extra child.
It does sometimes feel like a free for all! And one can only do as one would like others to do, but when friends arrive with unannounced sibs, I smile sweetly, know I have enough sandwiches, crisps and cake to sink a battle ship and get on with it. I expect that is what everyone else does really, which is why I felt the responses directed to anyone who might bring a sibling unannounced seemed a bit over the top.