Well, environmental components (such as the style of upbringing e.g. 'mix of freedom and strictness', and simply socioeconomic status, which affects quite a lot) are also quite heritable and shared between siblings, so we'd expect their intelligence to be even more correlated. However, intelligence is hereditary for the most part compared to that shared environment - for example, adoption studies show that the IQ scores of adopted children show higher correlations with the IQ scores of their biological parents than with those of their adopted parents.
Everyone seems to be quoting a lot but in the end it seems to converge to a handful (at least, I hope) studies with adopted children. I'll try to find the references before making judgements.
Until then, whether this is really useful or not, I want to mention that time and again my experience in academia is that topics like these where we really depend on the judgement of the investigators a lot need to be carefully evaluated. Academics have a tendency to over exaggerate significant beliefs from highly questionable data points from very few studies. I'll revert back after going through the literature best I can.
In addition to adoption studies, a lot of analysis of heredity can be (and has been) done in twin studies, comparing and contrasting the variation between twins and "ordinary" siblings; and fraternal vs identical twins.
You're right in that there is a limited number of studies, in part because you're looking at small populations and you don't get that many new adoption or twin cases each year. However, in that regard it's worth looking at equivalent studies worldwide who each have tested their local adopted or twin children; as far as I understand, these studies in different countries generally confirm the same or similar results, but I have not personally gone in detail to verify that.
