a good friend who has sort of invited her sister to a 30th birthday drinks and cocktail night, to put her off.
The sister is just an acquaintance and not a good friend, and to be honest, would put a damper on the whole evening.
if it would make your friend happy, why not go with it, if you definitely don't want her , then best to just say to your friend you hadn't bargained or catered for more people?
Is she that bad? Do you actually dislike her? Hopefully there will be enough people there to "dilute" her. You're in a bit of a tricky position! I would let her come.
You might offend your friend if you tell her that you don`t want her sister there. I would probably just go with it and steer clear of the sister if she bothered me. You never know, she might liven up after a couple of drinks and be OK
Could invites be gone to those invited which states that it is invite only due to booking? Maybe more if a special area is being booked like a booth in a bar etc...
Are others likely to invite other people too, to run the risk of it being more obvious?
well It depends on whether you want to keep her as a good friend and how good a friend she is now. A really really good friend would understand that she had misunderstood.