I'm a twin. My sister was born 10 minutes after me.
My older two sisters were born over 3 years earlier; they are also twins (They are identical, while I am fraternal with my sister).
We became subjects in a series of studies of twins by the University College London (UCL) into this subject. How much is genetic; how much is environment.
My older sisters carry the same genes and had a very similar upbringing. Any differences in them provide a control as to the effect of different experiences, even in the same household.
My twin sister and I carry different genes, and also had a similar upbringing, so differences (there are very many) are down to genetics, combined with the environmental differences noted in the older twins.
After years of research, it came down to 'A bit of both' The point is that 'a bit' varies with individuals. In some cases nature is stronger; in others environment.
Here's some recent results:
https://www.uq.edu.au/news/article/2015/05/nature-v-nurture-research-shows-its-both
and some discussion of the UCL studies:
http://blogs.ucl.ac.uk/bsh/2016/08/05/new-evidence-supports-the-use-of-twin-studies-to-explore-the-effects-of-nature-and-nurture-on-human-behaviours-in-childhood/