In the original idea for alien by Dan O'Bannon and Ronald Shusett the five crew were all male, or at least they were written as if they could be male or female, with little sexual colouring, gender having little importance in the future.
The choice of making Ripley female, and thereby ensuring that the last lone survivor of 'Alien' would be a female must have been influenced by Ridley Scott. Many of his films have strong women characters -- Someone to watch over me, Thelma and Louise, and GI Jane.
Only then was it possible to cast Sigourney Weaver in the role, and IMHO without Ripley being female, and without Weaver in the role, the sequels probably would never have been made at all.
One more thing, among seven Oscar nominations for Aliens Sigourney Weaver got a nod for Ripley. Although she didn't ultimately win an Oscar, it is little recognised that, in the entire history of the Oscars, no actress had ever been nominated before in an action film, a sci-fi movie or horror picture.