Here's an interesting one:
Older Sibs May Not Necessarily Be Smarter
They blame a faulty previous study because they "tended to compare children from different families". Thier study instead was a "large nationwide sample."
Anyway, how can you get any definitive answer anyway. The only way to do a direct comparison, is to test a younger sibling when the reach the age of the older sibling (a min of 9 mo). And then the conditions will be drastically different.
And if you take a sample of older and younger kids at the same time, then wouldn't the teaching methods for the younger kids be different then the older ones from when they were that same age?
I don't think there can be any conclusion from this.