The Blunt One · NOT WRONG
A seat limit is a real limit. The person who planned the meal gets to decide whether the table can absorb another guest.
Editorial verdict preview · etiquette
I planned a small dinner at home and invited eight people because that is all the table fits comfortably. The day before, my cousin asked to bring someone I have never met. I said I could not add another place this time, and my cousin replied that a warm host would make room instead of making guests feel unwelcome.
The question
UR WRONG separates the two accounts from the crowd's judgment. Read the frame, make your call, then compare it with the live jury.
Three editorial voices
A seat limit is a real limit. The person who planned the meal gets to decide whether the table can absorb another guest.
Your cousin should have asked earlier, but a flat no can land as rejection when the request is tied to a relationship or a difficult evening.
Plus-ones are not automatic unless the invitation says so. A clear capacity rule is kinder than pretending the arrangement can stretch.
Invite the other side, keep each account separate, and let people who know neither of you decide.