C.B.
Where we live (Groton, MA) there is no question that a play date is ONLY for the child invited of the same age as my daughter. I would NEVER have an uninvited sibling stay, unless it was pre-arranged with the mom that she had to run an errand or something. I have 2 daughters who are totally not interested in their friend's little brothers. However, as you pointed out, if the parents stay to talk to you as a friend or to get to know you before leaving their child with you, then siblings need to stay also. We have avoided the annoying little brother or sister issue by all meeting at the playground or library. My kids can see their friend, I can chat to the mom, but I do not have the responsiblity of keeping little brother happy and out of my kids' hair.