You're asking two seperate questions there.
Were women treated fairly?
Yes because in the times the westerns were supposedly set in most part the woman was the homemaker and didn't have high profile jobs. The highest would likely be a teacher. If you were to reseach that time you would struggle to find women that have the same kind of fame/notoriety of someone like Billy the Kidd, Buffallo Bill, Jesse James. The only one that springs to mind is Annie Oakley. Maybe Calamity Jane, but she may be fictional purely made up foor the Doris Day film. This does suggest that in impact terms women were quite invisible, but of course their impact would be felt in the roles they did have.
Are they sexist?
I'm not sure where you are going with this one. The answer above stanRAB and as Recordplayer says it has to be in context of the time. This goes for both in film and in the real life. It's not sexist to have women being less strong than men as in those times women weren't becuase of how society was so it's not that innacurate a portrayal anymore than it would be if women in Victorian drama were seen as being inferior to men. Back then women didn't have the vote, after they were married they often did become the housewife and rarely a mother two goes out to work.
If you are talking about the amount of screentime, they get, lack of character development then I would say no because it was across the board in films when westerns were being made. Whether it's gangster films, science fiction, war, police or horror women were never given big roles. I think it isn't because of sexism but more because back then almost all of the writers, poducers. directors were men so would write from a male prospective and have male leaRAB. This wasn't just in films but on TV and in books. Things are changing as more and more women are achieving high profile roles in the industry so things are getting more balanced.
However that isn't saying that things have changed that much. If you pick up books by female authors virtually all have a female lead character. They are doing the same thing men were doing back then and continue today. Writing a lead with their own gender and their point of view, even when it's unoriginal and cliched. For example the woman being held back in her job because she's a woman, the sexist boss or employer etc. Yes of course that happens, but it's creating stereotype male characters which is basically what the women in westerns were. Weak, home maker, neeRAB saving by a man etc.