As a mom of 2 and owner of an indoor play area & party centre I have LOTS of experience of 'party extras'.
When you have a party at a location, you are charged by the head - my play area charges only for the children, but the other in our area also charge for the adults. One solution I have found, as a play area owner / manager is to get the party parents to provide a list of the 1st names of the invited children. We ask on arrival if they wish to pay for siblings (some do). Then when 'little Johnny' turns up with a sibling, we are able to say 'okay, Johnny is on the guest list, if your other child is staying there is an admission fee of $XX'. If you are having a party at a location, maybe suggest this.
I am having my 'DS about to be 6' party next week, and i am surprised at how many parents have either not replied (don't they know what RSVP means?) and of those that have several have asked to bring siblings along. They do not know that I own the play area (he just started kindergarten)! The party is 5-7pm so I am providing dinner for the kids, and now have to provide extra for the siblings too! It hasn't even been a case of them saying they don't have anyone to watch the other child, just that it would be fun for them to come too! I already have the whole class invited & 11 friends!
Where I live (Montreal, Canada) once the kids are in school the majority of parents 'drop & run'. (Some don't even want to speak to the hosting parent - they just want to send them through the gate & leave!) Most seem happier to do this in a more public place (such as a play area) as there are more people around & it is much less likely for anything untoward to happen.