Family blood lines used to be considered really important until about 100-150 years ago so incest was rather common....usually only between cousins but siblings wasn't unheard of. It isn't a good idea due to amplifying genetic defects and is considered gross today but that's the past for you.
Thanks CDC
by
500k is within the reason of believable at least due to disease and hard conditions at least (probably still too high). 6M++ makes no sense for a huge number of reasons from the virtual impossibility to the census counts