Simple, the idea of Leia being Luke's sister wasn't added until the second draft of ROTJ. When Yoda mentions "another" he's not refering to Leia he's refering to someone else who was supposed to be the main character of the Ep7-9 trilogy. However when Lucas decided he didn't want to do 7-9 he copped out and made Leia the "other" and totally ruined ANH and ESB. Worse, he pretended that was the story all along.
As for a fantasy explaination, maybe Luke and OB1 were bait for Vader and Palpatine, Yoda was hedging all his bets on Leia so he oversaw her life and blocked any evidence of the Force. Could explain why Yoda was so reluctant to train Luke and why OB1 didn't seem to know about the "other."
OB1: That boy is our only hope.
Yoda: No...there is another
If OB1 knew about Leia then his line would've probably sounded more like:
OB1: Oh well, scratch that one where's Leia?
However, this whole theory hinges on whether OB1 knows who Leia really is, which I gather will be revealed in Ep3.