From a historical perspective, the much younger "siblings" of a teenage daughter and six month pregnancies were pretty common fictions to cover up unexpected pregnancies. So were nine month visits to "an aunt in the country" for young women.
There were cases where the child of a couple was actually their niece or nephew - the parents had too many kids, and the aunt and uncle couldn't have them (or were better off) so they adopted the child. Plus a rate of children with falsely listed fathers that is a *lot* higher than we would think at first, particularly with no DNA testing. And kids who were adopted, but the adoption wasn't talked about. Or kids from a short first marriage who were folded into a second marriage and treated by everyone, including the kids, as full siblings of younger kids.
These things were a lot more common than I think we realize - limited options to control fertility and strong societal punishments for out of wedlock children didn't mean people were always chaste outside of marriage, or never had affairs, but society developed little tricks and agreed on deceptions to cover up or smooth over problem, so that they could still keep the community running reasonably smoothly.
So basically, it's pretty much guaranteed that somewhere in an extended family tree of a few generations there some sort of situation where the documentation doesn't match the biological reality.
Interestingly, in the country I live in, birth certificates *now* have entries for the mother and the mother's husband. If the father is not married to the mother, he doesn't go on the birth certificate.