Identical twins reared apart are much more similar than fraternal twins reared together.
And to me at least it makes a lot of sense. If you share almost 100% of your DNA with someone else, I'd expect you to look the same on the outside - that's a given. In the same way I'd expect your brain to be highly similar and therefore it makes sense that you will behave similarly in a lot of situations even though you were reared in completely different environments. I mean you're not gonna get a different color hair just because of environmental differences, so applying this to brain development is not such a stretch.
But why do you think the structure of one's brain affects personality?
We have these things called universal computers. Differently structured CPUs run the same programs. It doesn't matter so much which CPU you have, they all have the same capabilities.
Why should it be different with brains? I think brains are universal knowledge creation devices, and universal means universal, structural changes won't alter the feature set.
The brain is not a general purpose computer running the "me" program. The brain _is_ the "me" program - the software is in the structure of the hardware. Different software yields different results. IANAN(...euroscientist) though, I just like reading Oliver Sacks et al., so take that opinion with a pinch of salt.
There's lots of research that shows that the structure of the brain IS very important in how it works and how we use it. For a good overview see Steven Pinker's How the Mind Works.
Many of these studies include individuals that had isolated damage to one small part of the brain and how that affects them. They show that specific parts of the brain are involved in highly specialized activities and that the brain is not just a big, nebulous general purpose calculator.
And to me at least it makes a lot of sense. If you share almost 100% of your DNA with someone else, I'd expect you to look the same on the outside - that's a given. In the same way I'd expect your brain to be highly similar and therefore it makes sense that you will behave similarly in a lot of situations even though you were reared in completely different environments. I mean you're not gonna get a different color hair just because of environmental differences, so applying this to brain development is not such a stretch.