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”.
Parent-child incest has the power dynamic issue. It’s basically impossible to consent in that relationship. As to siblings, I’d argue that the logical conclusion is that it is probably okay, unless there’s a limit to how much birth defect risk is allowable, which as noted above, comes with other issues.
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.