Ripley's gender may've been unspecified in the script for Alien, but that doesn't apply to Aliens.

And Liea being Luke's sister was an ass-pull after Lucas decided he didn't want to do nine movies and needed a way to quickly explain the, "No, there is another" line. From what is known of the third trilogy as planned in the late '70s, the plot would've focused on finding the alternate after Luke failed against Vader.