restaurant scene, weird?

SPOILERS
So John is having a dinner with his sister and before they order anything there is a bottle of wine on the table, but the bottle is already about 1/4 full and John is pouring himself another glass, after that he is offering to pour a glass to his sister, to which she replies that she had enough to drink at the wedding, moreover her glass is clean. So I was wondering did John drink 5 glasses of wine before offering some to his sister or did they ordered already half empty bottle?! WTF?