The Five

By the end of Dornford Yates's Berry series, Boy Pleydell is both first cousin and brother-in-law to both Berry Pleydell and Jonathan "Jonah" Mansel. Jonah's sister, Jill, has married Boy whose sister, Daphne, is married to Berry. Thus, these five grandchildren of Bertram Pleydell comprise:

two brother and sister pairs;
two married couples;
one bachelor -

- yet remain just five people.

They have no children. The family turns in on itself and ends. They also consider that the world in which society has changed and in which they have been unable to maintain their ancestral country mansion is not worth living in.