Twins-vs.-siblings studies are irrelevant to variation across large social groups. There's no way anyone can "write with confident language" about any of that variation being biological in nature, given that the social milieus are so starkly different.
Twin vs sibling studies aim at quantifying what effect genetics has on IQ rather than environmental conditions. If there are hereditary components to it, then they most certainly can be mapped to some extent across population groups, as have a lot of other "traits". There are only so many ways to slice this, but it is unfortunately a delicate topic.
