Ripley did want them to quarantine according to procedure, and the only reason why they didn't is because Ash disobeyed her due to his secret orders from Weyland-Yutani...
...which is something Ripley confronts him about. Also, Captain Dallas later says that they're all going to have to go into quarantine anyway when the Nostromo gets back to home base, and they jettison Kane's body after the chestburster pops out of it, reducing the amount of Xenomorph contamination still left on the ship, so the movie's quite consistent and believable in that regard.