Flicka, thank you! That's very, very helpful. In fact, it's been very helpful to work this out here and get input. I hope other people can get some help on their own conundrums, too! Don't want to dominate the thread.
I think I might have been unclear with my "he". I'd meant the son might get a courtesy title, not the husband, but I think neither of them would anyway.
[Also, luckily, dear mummy is quite old, so a very old title is exactly what she would have (the sole fantastic element is that these people live very long lives--a secret they keep to themselves, naturally).]
eldergrantaire, I'd imagine it would go to the eldest daughter, or the eldest daughter of the eldest daughter depending on the generation. So if Baron Featherweight had only two daughters (A and B), the eldest would inherit. But if those two daughters each had a daughter (X, A's daughter and Y, B's daughter), and both Baron Featherweight and both his daughters were dead, it would go to X, as the eldest daughter of the eldest daughter, even if X were actually younger than Y. (It's possible for the younger sister to have a child before her older sister.) At least, that's how it would work if it were males, so I'd assume the same pattern would apply. Someone please correct me if you know otherwise! ETA: benbenberi set me straight below--ignore all this nonsense I wrote.