It's a common misconception that Sabrina is first because the names of the badges between both Gym Leaders in question seem to have been mixed up during the text programming. The 'Soul' Badge would be more fitting for a Psychic Gym Leader, while the 'Marsh' badge more fitting for the Poison Gym Leader. The fact that they are the other way around is probably what makes people think otherwise, along with the way people play the game, moving straight from Celadon City into Saffron.
If you think about it, the way the game i layed out, you arrive in Lavender Town, go to Celadon to retrieve the Silph Scope, defeat Team Rocket at Pokemon Tower and get the Poke Flute, then head for either the Snorlax in Celadon, or the one on the route below Lavender, that you have probably encountered while exploring and training. Following on from that, the next place you go is Fuschia.