Midwives have been told about the benefits of “close relative marriage” in training documents that minimise the risks to couples’ children.
The documents claim “85 to 90 per cent of cousin couples do not have affected children” and warn staff that “close relative marriage is often stigmatised in England”, adding claims that “the associated genetic risks have been exaggerated”.
Siblings definitely have power dynamics that make consent very hazardous. I’d argue first cousins also have such dynamics. Perhaps to a lesser degree, but there’s no real benefit from having cousins marry and there is an increased risk of birth defects, so better to disallow it.