Shiraishi is 52 (she says that she's twice Akiko's age, back when they meet) and Kazumi is 16 (I'm not sure that's specified, but she's in her second year of high school. The school year in Japan starts in April, and Kazumi hasn't had a birthday yet.) Push things, and Shiraishi is old enough to be Kazumi's grandmother.

On the other topic, I think I'm happy for people to be a bit confused by that transition at first, as long as it becomes clear by the end of the entry.