knspol
Sorry but don't understand this. Did you pay any extra for the extra single cabin? If not then why should there be any refund? You didn't like the original cabin for 3 people, so you were given an extra single cabin albeit of a lower standard. I think you were lucky to have been given an extra single cabin and see no reason for any sort of refund unless an extra fee was paid.
What OP is basically saying that her sister had an inside cabin without a porthole, which would have cost £680 less than her sister's 1/3 share of the outside cabin with a porthole did. The sister therefore quite rightly feels that any refund should be hers.
This is depending on whether she has taken in to consideration that the outside cabin may have cost more with just 2 occupants rather than 3, to calculate it properly you need to take everything in to consideration. As we have not got the brochure or further details we can not do the costings, but why should the sister pay for more than she got, particularly as she was left by herself with no company, even though a lot of us would maybe have preferred a single cabin, including me but I would have to have a porthole to stay in it. Personally I do not consider a refund is due, if anything is due at all to the sister it should be the OP and her friend who paid the sister the difference between them as they benefitted and decided to stay in the best room.