Nature or Nurture?

What do people think is the biggest factor in creating someone's personality? Nature- genetics and such. Or Nurture - life experiences and childhood.

The reason I ask this is because I know an old couple, friends of my grandparents, and they have three grandchildren. Their family is fairly normal, parents pretty good all things considered, grandparents involved and loving. I bet they made mistakes and so on, but as far as anyone knows no major red flags.

Yet the three children have SUCH different personalities. One is a simple guy who is going to make a fantastic boring accountant dad one day. Definitely autistic. But the other two... One is just... toxic. Everything she does is just bitter and twisted. Her parents actually had her tested, part of a psych profile after some serious misbehaviour at school. Nothing diagnosibly wrong with her. And I'm left wondering why she turned out this way. (Often parents are the first looked at, abuse or neglect ect, but none of the signs are there)

And the third child is just so quiet. Loves nature and avoids people. Complete difference from brother and sister.

All three had a very similar upbringing, with no noticeable gigantic differences to cause such dramatic shifts in personality.

I'm naturally on the side of Nurture, but cases like this make me wonder just how much is almost pre-determijed by our genetics. How we process the world, literal sensory processing differences in brain etc.

Wondering at others thoughts on this?