Good points in your last paragraph. Relations between adopted sibs would certainly be creepy at best, even if not illegal (not sure what the law is in any jurisdiction, about marriage in such a case). Someone once told me that the reason the King of Siam dies at the end of Anna and the King of Siam (besides that is what actually happened) is because in that era, couldn't have a white woman falling for a non-white person, king or not.
For people who are very into their ethnicity (a.k.a. racist), it is considered "sort of" okay for a man of their race to have sex with someone of another race ("boys will be boys," "sewing his wild oats") but not the reverse (even when not actual lynching him for it). Although everyone is polite, I know some of Nom's family doesn't really approve of her marrying white.
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For people who are very into their ethnicity (a.k.a. racist), it is considered "sort of" okay for a man of their race to have sex with someone of another race ("boys will be boys," "sewing his wild oats") but not the reverse (even when not actual lynching him for it). Although everyone is polite, I know some of Nom's family doesn't really approve of her marrying white.