Sheldon Dan asked about marriages between relatives and whether it was common.
It certainly was in Poland. My wife's parents were first cousins and between 1800
and 1900 there were at least six intermarriages between their families. So
practically everyone was a cousin - One ancestor even married and had children
by two sisters - after the first one died. It makes for a very interesting
and complicated family tree and an even more complicated method of entering
them into family tree software.