That is easy 1979 or 1980 based on conjecture. Without solid evidence of course. The First star wars treatment was no earlier than 1973 and the rough draft script in 1974. I also don't believe He would have had that in mind in 1975-1976 and the idea could really not have struck him until the devlopment of the empire strikes back, Started in 1978.

Leia being the sister could not have been in his head in 1977-1980,I don't think. To about sometime around 1982, possibly 1983.

The Journal of the Whills document is about Mace Windy, and talks about someone called CJ Thorpe. The Sibling twins thing did not happen to the draft where Luke has a brother. At one point Luke was a girl. Perhaps the character was cut in 2 and divided into seperate characters, The girl Luke must be the first seed of the genesis of Princess Leia.