It is complicated but it still is no go. The rationale against it still stands on the grounds as you touched on.
1. Mistake
2. They may choose to have kids.
And there is something else, small but profound. Incest and other such relationships nurture introversion in society. You want a open looking people with investment in wider society. Introversion ( I do not mean the meaning in it's normative sense ) is not healthy for society. Thus far civilization has helped us to move from - me, to family, to clan, to tribe, to state and increasingly global sense of humanity. Incest is movement in opposite direction.
Indeed it was. No idea if you did O level history but we had piece on American Revolutionery War. Some of those fighting had only recently migrated from England or sorry settled there. Others were one generation distant, other couple more but they were all from York, Birmingham, Bangor, Manchester, Sheffield, Watford, Dover, Cambridge, Exeter etc
I mean George Washington????
Washington, Tyne and Wear, UK. Wonder if he came from this cold, wet, windy dump in the north.