I know it's too late now, but I'd have asked the parents for the extra $25 their extra daughter cost you: they don't know how many children are the minimum, I'm guessing, and you didn't invite the sibling. Let them know it's rude! (But then I'm a rude person too, just that as DS is an only I've never even considered doing the sibling party thing and wouldn't after being on BB so long.)

Glad the kinder thing worked out for you and your DD had a great birthday.