I mean, Jess wasn't family, was she? Not blood family. What if she had lived and gone on the road with the boys?

Wasn't the point that Sam was on the point of asking her to marry him? At which point she might well have been considered family (since I don't think SPN has the jaundiced view of marriage that Alias does).

I agree that it might be interesting to see a new female character interact with the Winchesters, but I also like their wretched family dynamic very much, and would want to see most of the attention remain on that.