Hello

I know this is a sticky situation - from both sides!
I am the mother of two girls who are 13 months apart. I didn't plan it that way, but it has worked out really well.
Being a Native New Yorker (Bronx born and raised) believe it or not, I would NEVER drop my 5 year old daughter off ANYWHERE. We're pretty liberal about life up here, but the life of our children is priority. I don't think I just dropped my girls off at a party until they were about 10. And I can tell you, 99% of moms up here thought the same way. It wasn't because we were bored and had nothing to do, either. I just didn't trust any other adult, especially ones with which I were only aquainted, with the life of my girls. Who knows if one of the employees is a pedophile (maybe someone who's a pedophile that's never been caught, therefore, no record)? Who knows if a cousin, an uncle or aunt, a brother, has a motive to hurt my child? I worked in the courts for the City of New York as a court stenographer and I've heard some pretty horrible stories first-hand at least once a week. Most involved cousins, brothers and uncles. Sorry, not being prejudicial, I'm just telling you what I've seen... tortured young children testifying about sleep-overs and parties they've attended and been abused at. Horrible. I wasn't ever going to let that happen to my girls.
Even house parties, the parents all knew if the kid was coming so was the parent. That's just the way we did it.
Now as far as siblings and parties. Everyone that knew me knew my girls come in a pair. While RSVP'g to a party, I would tell the mother that I have no family in the area to watch my other daughter, and if she didn't mind, I would be more than happy to pay for my other daughter to participate in the party. Both my girls would even bring separate gifts. Like I said before, my girls are only 1 year apart, and they were/are friends with each others friends.
Those were just my experiences and opinions. If someone wants to flame me, whatever. Not gonna change anything.
I don't think I've had a house party since they were babies, I've always had it out because it was much easier and I would let all my parents know if they wanted their other child to participate it would be say, $15.00. As the parent of multiple siblings, I offered that option to other parents with multiple siblings. There were a few times where I rented out a local movie theatre (not as expensive as you might think!) and I could invite up to 25 kids. That was perfect - no one was left out and even the parents enjoyed it. We saw two of the Harry Potter movies and two Disney movies. It was easy, everyone was involved, popcorn, candy & soda for all and no tears.
If the party is being held somewhere that can only accomodate a certain amount of children, I would let my other daughter know there is no room for her at the party, I'm sorry, that's the way it is, and I would have that daughter pick out small toys (polly pocket, barbie, etc) and coloring books/crayons, whatever, to keep them occupied while the party is in progress.
Even though I was straight-forward (which some people don't like, but that's who I am) I was always polite about it and if the parent made it clear that my other daughter couldn't participate, I respected their wishes, but me and other daughter were still in the building with my daughter. She never left my sight.
So maybe when these people call to RSVP, make it clear to them, I just want to let you know this place can only accomodate a certain amount of kids, and I know you have other kids and if I could accomodate them, I would.
Being straight-forward but polite is the way to be, black and white. Any shade of gray can be interpreted different ways by different people. When you are honest with people, no one can fault you.
Good Luck with the party and have fun
