While success and IQ are almost certainly correlated, it is not possible to make a statement like that.
You can just as well say that success comes down to height, or country of birth, or combined wealth of uncles.
There are so many other factors at play.
I think it arguably more likely that acceptance into the education system comes down to IQ due to antiquated practices in attempting to derive intelligence. Being within the education system is the sunny uplands to networking and generally better opportunities.
Its possible that almost every child has the potential of a "good enough" IQ and that nurture reigns supreme over the supporting cast of genetics. To separate the two within nurture there remains environmental factors such as an interpreted narrative of favouritism which can lead to downward spirals.
2. Same mother doesn't mean same fathers. Success comes down to IQ. One kid could have been lucky while the other drew the short stick.