I'm curious about a few things, including:
1) The general age of your players
2) The age(s) of the uninvited sibling(s)
3) The level of distraction/inappropriateness provided by the siblings
4) The relationship between the invited player and the uninvited sibling(s)
5) How long do your sessions last?
4. Well they're siblings. I think the older one doesn't really want to bring the younger one but is asked to by family.
That explain more things. Your friend dont bring his siblings to bother you but because he has no other choice. Not telling you in advance wasn't polite but knowing you would most likely said: "No, dont bring your brother", I can understand why your friend did what he did. Your problem isn't about unexpected players, it is about having a player who need to babysitte his siblings during your game night.
You must accept the problem and try to find solutions with your player unless you want to kick him out of the game. Maybe try to Schedule your game night when your player dont have his siblings with him? Prepare other occupation like movie for them? Include them in the game even if they aren't the best roleplayers? In my experience, kids love roleplaying generally.
As for your second problem, Cut down the number of players. You aren't gonna survive with that amount of PC and lack of experience. You will be burned after the first sessions.
Edited by vilainn6