Last year I had a Christmas party for my mindees and I said to the parents they can pop in for a drink, too if they wished. One mum and dad arrived with older sibling. While i FULLY, 100%, DEFINITELY AND ABSOLUTELY appreciated that he is ASD, the older sibling's behaviour made the party in my teensy tiny house very difficult (he screamed at everyone because we were not listening to him to mum asked us all to be quiet so that he could talk, he boxed and kicked a mindee because mindee had a balloon he wanted..... etc).
Today, mum is going to drop the mindee off at mine (it is not mindee's usual day, but I have obviously invited her). I am terrified that older sibling (who has not been invited as it is a mindees only party), will let himself in as he does, and then mum will ask if he can stay or then mum will just plonk herself down and stay with older sibling.
How can I politely and kindly and FIRMLY say (if this happens) that it is a mindees only do without upsetting anyone???