Thank you all for your helpful replies. I could not book and pay for my sister's tickets on Singapore Airlines website because the travelling party must have a credit card and she does not have one. Also, the special deal is only available for booking via the internet (with a credit card) or at the local SIA office (cash). My sister went to the local SIA office, booked and paid for her tickets there. This is why (a) I was not able to retrieve her booking record via SIA website; and (b) I could not locate her booking record on any of the 4 websites mentioned (virtuallythere.com, viewtrip.com, mytripandmore.com, and checkmytrip.com) - using her PNR on the ticket. It would have been good to be able to look up her booking online!