Whew!! Sounds like what we really need is a week-long workshop for folks who, like ourselves, are trying to shape our research into novels, memoirs, etc. I just want to say that I think this entire website is wonderful, because we black folks are reclaiming, shaping and publishing our history. We are in the process of rescuing our ancestors' stories and lives from obscurity. Isn't that a blessing?

As for your question, I'm going to recommend a couple of books. One is "Your Life As Story." It's a how-to that helped me figure out structure for my book. (My editor helped me figure out the rest.)
I also liked Generations by Lucille Clifton. This is a small book that actually tells two stories: her family's history and the response to her father's death. The stories alternate between chapters and come together at the end.

I'm suggesting these books because the presentation will determine whether to start with your oldest ancestor. Perhaps you should also think about your audience. If you're writing for the public, (e.g. Cane River) you'll use a different structure than you will if you're writing for your family. If you're writing for the public, you have to put your family's life in a larger context in order to make the information interesting to folks who care nothing about you.

Lalita Tademy showed how her ancestors coped with a society that defined them by their enslavement as well as by the color of their skin. But if she was writing a history for her family, she might have included different material.
By the way, I wish this forum had been around a couple of years ago.