“them half-Arkansasans”
They didn’t go to a family reunion looking for a date.
I remember my Mom telling the hard “facts-of-life” that I couldn’t marry my cousin when I was 11. There are natural laws that prevent this coupling in all societies. I think that these people have learned a lesson the hard, hard way about learning the most about the person you are marrying before you marry. Ask and ask again and your life depends on the answers,
Both being adopted, having the same birth date, and looking alike may have thrown up some flags.
You are making two assumptions though. One, that both celebrated their birthdays on the true date, and two that they looked alike. My wife's brothers look very little like her and/or her sisters, even the one who shares the "coloration" of the brothers.
They may not have looked alike. Fraternal twins look just as similar/different as other siblings. I look nothing like one of my brothers.
But in many societies the marriage of cousins is allowed - in some, it is even favoured.