Jacob could ask her how she became a vampire and she can tell him what she knows.
I love Alice, though I do have issues with her in the way she treats Bella at times. Alice knows her visions are only possibilities, so there are times she acts on those visions that it drives me crazy because where Bella is concerned, her future should be based on her own decisions, not on Alice's ability to see the outcome of those decisions. But overall I do love Alice, and my issues with her are for another topic, another day.
To answer your question, how much do I want to see of Alice's story, I have to say I am in the minority in that I really want to see very little, if anything. Only the parts that may be necessary to understanding the story at hand, namely, the story of Breaking Dawn, of Edward/Bella/Renesmee/Jacob, of Jacob vs the pack, and the Cullens vs the Volturi. I think the time to tell Alice's story has passed, and should have been done when they were dealing with James, if they were going to tell it, as Alice's past was tied to James in a small way. But James is gone, and so too, in my opinion, is the the right time to tell Alice's story.
If there is reason to expand on Alice's past in order to support telling the story that appears in Breaking Dawn, great! Show me all of it, if it's necessary. But there are other storylines that can and should be told, in my opinion, that add to the story much more than telling Alice's story, much as I adore the character. Things like Irena and her motives. Aro and his motives. What logic and what process of decision brings Aro and the Volturi to Forks? How the rest of the Denali feel about the rift between Irena and the Cullens. How they really felt about Laurent. What, exactly, did the various Cullens encounter as they sought "witnesses" to the showdown? What was going on with the pack back in La Push and how, exactly, did that impact the wolves when Jacob, Leah, and Seth broke from the pack? What history does Carlisle have with Aro and with many of the nomadic vampires, and what is each of the nomad's history with the Volturi?
To me, these are the types of things that directly impact the story in Breaking Dawn. While I find Alice interesting and I like Ashley Green's portrayal, I don't really think knowing Alice's backstory adds to our understanding of her character's motivations in Breaking Dawn--not in the same way that Jasper's or Rose's story impacts their characters' motives like in Eclipse.
I just don't feel like Alice's story is absolutely necessary to the plot at hand. That, of course, is the writer in me. Nothing is needed but what is necessary. One of my favorite writers (and teachers) once said to me, sometimes it's best to just let a character be. We only need to know how she is, not why. I think that's true of Alice in this movie. As an Alice fan, I would say I want to see as much as possible of her story. But the storytelling, to me, must triumph over my own fangirl tendencies!
That's a double edged sword there my friend...I LOVE Alice about as much as the next person does, and I would really love to see as much and know as much as possible, about each and every Twilight Character, but I believe that in Alice's case I think that not knowing as much about her past, as we do about other characters, adds to her intrigue, the mystery that is Alice would all but disappear if her past was put to screen, or if Steph, added more to her story.
Look forward to hearing from you on this one Doc!!