Families' Eldest Boys Do Best on Tests - Forbes.com: "Boys at the top of the pecking order - either by birth or because their older siblings died - score higher on IQ tests than their younger brothers. The question of whether firstborn and only children are really smarter than those who come along later has been hotly debated for more than a century."
Reading the article it seems that they can have up to a 3 point advantage, but 3 IQ points just isn't all that much. You'd be hard pressed to work with someone over time and notice that difference.
I can see firstborn children getting extra learning from teaching their siblings about how to do things. But I have also noticed that younger siblings start doing things earlier because of that teaching or imitation.
IQ seems to work best in academia. The higher your IQ the better you do in school. That doesn't always translate to doing better in the real world.