If a couple is going out to eat?

the person who did the inviting unless prior arrangements were already made.

The one who invited the third wheel or the third wheel.

If someone brings a third wheel, they should either pay for themselves or be paid for by the bringer, since dumping the extra cost on the other party would just be ridiculous unless you worked it out beforehand.

my husband pays for all howerever.....If you take it amongst yourself to invite a 3rd party, be prepaired to pay! you should atleast offer. If a person does not even offer, she is probably really selfish and inconsiderate. you'd be surprised how the smallest things can tell you alot about someone

I think the woman should make sure it is all right with the one that invited her.Then she should pay for her sister's meal since she invited her.

You or her, simple as that. It's rude to expect the man to pay, he didn't plan on having her come along in the first place.