Is that the only place that stat comes from? If so, I'd have to agree with Cricket2--I'd hardly consider it reliable, for those reasons. Still, we do know that intelligence is highly heritable.
To the OP--I'd caution you to be careful about underestimating a second child who presents differently, First kids tend to have certain personality traits that align with a traditional concept of giftedness, and may tend towards more outward achievement, while laterborns may decide to take on different roles but be just as bright. Also, your second is very young. I have revised my estimates of my children's relative giftedness a fair bit over time, and may do so again.
