sounds like u did a great job on the invites. Now if an univited sibling showed up how would you have handled it?
It's not the (iunivited) child's fault that the parents are rude or clueless. I don't punish the child, to the best of my ability.
I always have extra goody bags on hand. They are not the same goody bags the invitees get, but more a bag a candy--serves the parents right for brining an uninvited guest!
There is always enough cake. And I always have a little extra food on hand, just in case. For the pool party it was a SpongeBob theme. We were serving "crabby patties", i.e., kid meal hamburgers. We picked up 3 extra meals. I also like having extras in case someone drops theirs or spills their drink on it, etc.
I have extra piñata bags on hand. OK, they aren't the cute, themed ones; they are lunch bags, but hey, you weren't invited! But it does mean less for the invited children--piñatas will hold only what they will hold.
As far swimming went, if siblings had shown up, and invited guests declined, they could swim. If not, I'd have to tell the parents, I'm sorry, but there aren't enough lifeguards since we provided only for my child's friends. That wouldn't be able to be helped. 1) It was $75 extra for every extra lifeguard. Really, I could wind up need 4-5 extra lifeguards if every child had all their siblings attend. 2) Lifeguards at the last minute would not be available. It has to be booked in advance. My numbers were due 10 days prior to the party for that reason.
Other times, I just pay for the child and think to myself how rude of the parents. We definitely would not be friends if they cannot show any consideration to me or to my child.
Really, my middle one isn't an overly social child like my oldest. He has friends, but it is not a wide group. With his parties, there is always room. He likes something small. But again, that is a reason that I don't like siblings to show up at his parties; it makes him uncomfortable and it is supposed to be his special day.
I do cheat. When the kids are in preschool, I hold the parties during the day when most siblings would be at school. It also helps me get a great discount on the party place!