Oh god I used to hate this - flipping sib gate-crashers.
For my Dds 7 th another mum and myself held a party in a sports hall for the entire class. It was a pretty organised gym lesson where they were all supervised by qualified instructors using quite full sized equipment.
One of the girls has a little brother and the dad brought both, and hung around the centre with the little boy. Some parents were coming and going- mostly dropping and running.
This dad studiously avoided doing any daddy duty, and his little uninvited son was unsupervised, fell, and cracked his head open, and had blood pouring out of it all over the place. The girls all were screaming. There was a lot of blood, gushing out, all over the floor.
I was so annoyed with the dad for a) just standing around in the way, talking about sport to some randoms and not just dropping his dd off,
And b) bringing his, much younger uninvited son to a girls party, and c) expecting some woman to look after him (just like at home).
The other mum and myself who were there in the centre had 30 X 7 year old girls to look after and honestly didn't have time to wonder what was happening to the little 4 year old uninvited boy who had wandered in and therefore had unsupervised (and uninsured) access to the equipment.
The little boys mum rang me later that night and apologised for her crap husband, and was mortified that her boy had caused such a flipping palaver with the bloody, gushing head wound, and the screaming.
He now has a massive scar on his forehead, which looks awful.
but I still can't talk to her normally, as before, as she knows her DH was in the wrong, (and that i think he was in the wrong) and that poor little boy has that massive scar to prove it.
It was a nightmare. And the other mum and i were terrified of being sued (even though the boy's mum rang me to apologise, later, they might have sued later down the line if their son needed skin expensive treatments, like grafts or anything).
Just a nightmare.
I think "no siblings please" is fine, also "sorry, no siblings" would do nicely. Enjoy the party!