Sweetness at the Bottom of the Hangman's Mustard

I just finished the third Flavia de Luce book -- "A Red Herring Without
Mustard". I've got a question I'm hoping someone here can clear up for me.

This book, as well as the first two FdL books, talk about Buckshaw having
been in the de Luce family for centuries. All sorts of dead de Luces are
named as having done this or that with the estate.

Flavia's father is Col. de Luce.

There's also some discussion about the property being owned by Harriet,
Flavia's mother. I think there's some discussion about her having grown up
at Buckshaw.

How is the property owned by Harriet if Flavia's father is the de Luce with
the aforementioned decedents?

Possibilities I've thought of:

- Flavia's parents are siblings/cousins and this is a case of
inbreeding amongst the aristocracy.

- Flavia's father took his wife's name when they married in order
to keep alive the "important" lineage.
(Was this a possibility in 1950's England? I have no idea when it
became possible in the US, much less anywhere else in the world.)

- The author is confused.

- I am confused. (Actually, this is a certainty and not a possibility.)

I don't remember this being explained in the first two books, and I'm pretty
certain it wasn't in the third.

Thanks,
Wayne