This is the smartest one I'll get

As a parent of an only child, I found this article to be very interesting. Basically, River is most likely to be the smartest of any children I’ll ever have. That might give me pause when thinking about having another child, except the fact that bringing another child into the family is what will raise River’s IQ that extra notch.
So:

-I can expect River’s intelligence to increase if we give him a sibling

-It doesn’t matter whether I give birth to or adopt that sibling.

It’s the process of River teaching things to a younger child that will develop his intellect.

This is a fine thing for River, as he seems to be fairly intelligent already (no parental bias there) and he has the prospect of another small increase with an expanded family. But what does it mean for the younger sibling? There comes a point where one child is going to be the youngest, no matter what, and won’t have opportunities to teach younger kids. What to do then? Find peer tutoring opportunities? Just accept a slightly lower IQ? Might all this be related to angst of 2nd and later borns at having to struggle to measure up? How do your experiences relate to what this study says?