Peninsula Girl, well done! A nice smile and being firm really makes it hard for these pushy parents to chuck a tanty
I have 6 kids and eldest is fifteen. so I have catered, hmm, 50 family parties including anywhere from 10 to 20 kids (immediate family and two best friends kids), and 31 friends parties. Yes, I did need a calculator for that, lol.
We have had lots of home based parties as well as many outsourced parties. I never pay for siblings to attend the outsourced parties, unless we have a cancellation and I can fit them in anyway. But I love parents to bring siblings, as long as they understand that I cannot always pay for them. I always do extra party bags for siblings. Or give them a lollypop or Freddo.
For home parties I really do not mind. We've all been in a situation where child care for our other kids is not possible, however if its me, I ask nicely about bringing another child, and like it if the party guests parents ask me, so I can do extra party bags.
The games can be a problem. If older kids attend as siblings, then they do tend to dominate and win most games. One mum always brought her older daughter for years, and for years at every party she won most games that were for kids 4 years younger than her. It frustrated the other mums, but none mentioned it.
I ended up, after the second year, making it explicit (as I had my own older kids, this was not just targeted at her) that some games were little kids only, then a couple the older kids could play. I gave them jobs like spinning the child for pin tail on the donkey, or doing the music for musical bobs. Worked well.
It's a minefield, this birthday party thing