Homosexual relationships dont inherently harm anyone, they're functionally just the same as celibacy at the level of government, having a kid in an incestuous relationship is open and shut child abuse, in a similar way that excessive smoking and drinking while pregnant is child abuse but worse ofc.
BBC article on the effects of first cousin marriage, not sibling marriage in the UK: https://www.bbc.co.uk/news/articles/c241pn09qqjo
Channel 4 documentary about the kids of cousin marriage in the UK from about 10 years ago: https://m.youtube.com/watch?v=gDGUZxUTwBI
