I thought it was common knowledge that Leia wasn't to be the 'other' that was referred to? Don't they say as much in the 'screenplays' book that came out (Oh and if you can, you should really look at the 'Jedi Council' roundtables on AICN - there's some great great stuff on there). Luke DID have a sister but it wasn't supposed to be Leia, and as far as I remember, an increasingly coked-up and badly behaved Carrie Fisher started making demands, one of them being that it would be a lot more intersting if she were made Luke's sister. In fact she is said to detail all of this in her book (Should be due out soon). It contains all of her diaries and tells of the making of all 3 movies.

Do you really think that if the plan were to have Leia as Luke's sister they would've had that forced love triangle in Episode 4? Granted it isn't a love triangle as such, but there's defiantely something there. It's abandoned more or less for ESB but then it's tonally completely different from ANH anyway.