Mrs. Frederic's Promise?

Mrs. Frederic has mentioned a couple times that she made a promise to Leena. Here's my thoughts: We don't know Leena's last name, and Mrs. Frederic's grandson is now very old. It's a long shot, but my cousin and my sister and I suspect that Leena may be a great great granddaughter of Mrs. Frederic, and the promise has to do with that? Thoughts? What do you think the promise is about?