Seattle does hold the tiebreaker with the Astros and yet, the Astros do clinch a playoff spot with one win, because the Mariners and the Rangers are playing each other (and they hold the tiebreaker with the Rangers. They also get a spot in the event of any four or three-way ties - with the Mariners winning the West and the Rangers out).
If the Astros win on either Saturday or Sunday, they're in. If they lose both games, but the Mariners also lose both of their remaining games, then the Astros still make it. If the Astros lose both and the Mariners win one of their last two games, then the Astros are out.
__The Astros are still in the hunt for the AL West title -- which would happen if they win both of their remaining games and the Rangers and Mariners split their final two games, because Houston holds the head-to-head tiebreaker edge over Texas -- and send Justin Verlander to the mound Saturday.