Sidney writes Juliet that her few chapters won't do; her book needs a single voice to ground it. He believes that voice should be Elizabeth McKenna's. Everyone spoke about her at some point and she's responsible for saving so many people. Sidney wonders how an art student became a nurse, fell in love with the enemy, and had a baby alone. He suggests that Juliet speak to people again and look carefully at Elizabeth's things. He also believes a story about Elizabeth would be invaluable to Kit.
The comment that Elizabeth had Kit alone suggests that while Sidney may now be a beloved friend of the Society chosen family, he's not entirely sold on the idea that chosen family can alleviate all the stress of having to bear and raise a child alone. He does, however, recognize that Elizabeth held the Society together, and that her story is compelling and will resonate with many readers.