Ignoring the bills for the mobile phone (useless to me) and a few letters from the cable TV company (wouldn't yield anything), I went straight for her credit card statement. I suppose I could have picked any, but I really wanted the most up-to-date. Hmmm, what's my sister spent this month...? Actually, I couldn't care less, but in order to fly to Chicago to see my friends' new child, I had to get my hands on this apparently invaluable piece of paper.

It all started when my sister Lib and her boyfriend agreed to be "bumped" from an overbooked United Airlines flight in return for a couple of hundred pounds-worth of flight vouchers, which were rapidly approaching their use-by date. Now, I had holiday time booked and friends in the Windy City who were way overdue for a visit, so Lib booked my flights for me, and used those otherwise soon-to-be-invalid vouchers.

So on a day off, she phoned up, explained that although the credit card was registered to Dr Elizabeth Pearson, it was actually Miss Rebecca Pearson who'd be flying. "Problem," she was told. "You'd have to be there to present your credit card." "Nonsense," she replied, "that's absurd, and what is more I shall be in Italy when my sister is flying." So, off trots the salesman, who comes back admitting he was wrong and that in true 21st-century style, all that I needed was a reference number.

Lib set off for Italy; my tickets didn't arrive, so I phoned United and spoke to two reps, who told me that the reference number alone would be sufficient, but, what the heck, they'd send me e-mail confirmation to make sure.

The e-mail sent me back to square one: the credit card has to be present at check-in. "But it's somewhere in the Italian countryside," I cried, and spent the best part of an hour going around in circles on the phone. Again. Eventually, the day before I was due to fly, I was told that a credit-card statement would suffice. So, with my sister's permission, I rifled through Lib's stuff until I found the time-and-motion-sensitive document. And when I checked in at Heathrow? All they wanted at United's check-in desk was my passport.

After the trip, I contacted United. The airline confirmed that the card used for payment has to be present for check-in, and insisted "this policy is strictly adhered to". United said that the staff "failed to inform" my sister of the policy, and sent me travel vouchers to the value of £75 "as a gesture of goodwill". I am now planning to return the favour to Lib and book her a ticket - but I will make sure she carries my credit card.