Politely refusing siblings at parties

I know this has been discussed in detail before ... bringing non-invited siblings to a birthday party.

However, a friend on Facebook just posted about parents she does not know (classmates' parents) emailing and texting asking to bring siblings. She's miffed and wants to know how to politely decline.

Is there a nice , subtle way to do this?

"I'm so sorry. We're at max capacity for the party."

Re: Politely refusing siblings at parties

I'm sorry but we are at max capacity.

I will let you know after I get all the RSVPs to see if there is room for the sibling.