I had a whole thread on a similar thing to this a year ago!! DS was turning 2 and we were going to a play centre (quite expensive) and one girl wanted her 5 yr old to come, very awkward cause he was/is a bully and laways hurt the little kids so I ahd to politely say no it was only for the little ones.

I know my class have had a number of whole class parties (I teach kindy at school) I haven't heard of any probs with siblings coming but I can ask after the holidays if you like?