I've been to a few playcentre birthday parties and the done thing is to bring along the siblings but pay for them seperate. They usually have a list of the invited kids to check of anyway and the person at the door will tell you that only those kids get in paid for by the party host so the others will need to be paid for.

When we had A's birthday last month we had 2 no shows who I had to pay for (Grrrr) so a brother ended up joining in the party and having some food and cake anyway.