At age 5, I leave DS at parties at sports centres, farms etc. It is expected - the parents are very much "hi, hi Liebling, you can pick him up at X time". If he's clingly then I'm told I can stay if I really want - code for I'm not really wanted.

I would not expect siblings to turn up to a party where I pay for places - unless the mother took them off and they didn't join in the party.

When I have done a party at home, siblings are welcome. Because I invited only a few and Liebling knew the siblings. I think now he's at school I don't know if there would be room for siblings, and the only home party he has attended since we moved I was expected to leave him there too.