That seems like a very narrow focus for a campaign. I wouldn't go very far with it other than to say that you can get one feat if you have at least three ranks in the skill, and then leave it at that. There's no reason, really, to be a legendary knitter. Anyone with a few ranks and time to take 20 on a Craft check can probably call themselves a competent seamstress.
Maybe you could give a second extra feat if they have three ranks in Profession (seamstress) or something, too. Plus, they can make a few extra copper pieces on the side.
Other than that, it's really more of a starting concept rather than a "we continue to progress in our knitting and combat abilities in lockstep" or something like that. There's very little roleplaying potential in being a group of needlepointers. It's really just a background element.