IMHO, RSVP is out of date with modern society. After all, if I can't even get my own family to RSVP properly, how the %#!! can I expect strangers to RSVP. It seems that we have SO many things to do in modern society, that we just don't seem to have enough RAM in our brains to remember to deal with yet another line item.
The way it works for me is to RSVP immediately. You see the invite, look at your calendar, reply. If you're going, add to calendar. Done.

have a family that puts me on the spot every year and says: "Can I bring her brother, we have no sitter, etc." I am not friendly with this family, but I always feel bad for the girl and say OK. Three years in a row I paid extra for the brother and then the whole family no shows and never mentions anything about it.
I don't do siblings because if I let one in, it hurts feelings of those who went to the trouble of getting a sitter for their other kids. I say "I'm sorry, we can't, we're already at the limit and the venue doesn't allow extras for safety reasons.... let's set up a playdate instead."