The other ended up becoming Leia. In Jedi Yoda whispers on his deathbed "there is another Skywalker"
Who knows what the original plan was.
I have read sort of other ideas vague notions of Luke having another sibling that is not Leia very far and remote in the galaxy, largely coming from something Kurtz one said.
There is also the idea that Hamill's character could be replaced if the actor was difficult or they decided to kill Luke off.