Pure anecdote, but I and my sister (2 of 2) were born almost exactly two years apart, and I know that before she was born, there was an absurd amount of attention paid to me for the first couple of years (judging by the sheer balance of photography.) I was also read to constantly, and learned to read fairly well before kindergarten.

By the time my sister was a couple of years old, it was me that was constantly reading to her. The age distance there allowed me to be able to teach her things as well as my parents, and I'm sure that the act of reading for her and showing her things helped me to strengthen my grip on those things myself.

As adults, we're both 99 percentile types now, according to IQ and standardized tests and such. I'm not sure had the dynamic been different, we would have turned out the same way.