Interbreeding is as old as mankind.... it's only since the 'rules' were written out in the Bible that people made an effort not to sleep with their sister's husband's aunty's son. Mind you, that doesn't stop some people....

It's been legal to marry your cousin for over a century; it was made legal to marry your husband's brother after World War I decimated the male population and 'breeding males' were low in numbers. It was made legal to marry your wifes' sister before WWI but I can't find the reference.

Consider the estate to be a village - put that village on the top of a moor, in a deep rural valley... you've got to breed "o'er the midden" if you want to have any chance of procreation at all.

(Better marry o'er the midden than o'er the moor = marry someone close, over the muck heap/farmyard rather than someone from far away.)

LTS