Anonymous wrote:"Due to space restrictions at the venue, we regret that we cannot accommodate any siblings." And if you want to and the kids are old enough, you can add "Please feel free to drop-off for the party."
Something like this or the first suggestion. I like the explanation for the no siblings because people will understand why the policy is in play. Sometimes I think people ignore the no sibling because they don't get why it is there, other then you asked for no siblings. We have friends who have twins and were surprised to get the no siblings invite, normally both the boys are invited because they both know the child, but that has started to change. I think it is nice when the parents explain why because it allows you to say to the other kid "there are space limits and Johnny does not know you like he knows your brother. I'll take brother, what do you want to do with Dad?"
I guess I find just "no siblings" a bit cold.
But everything you send about the party needs to include the reminder of no siblings.