I would just look at those same twins you mention as disproving the idea that "nature v nurture - nurture won"
Otherwise they would have stopped, because of lack of need, using identical twins for studies wouldn't they?

Of course, that there are plenty of other factors too, which is what the studies you mention may show, isn't in doubt.
That said, if there is something I can do that would make me good at piano I don't know what it is...I've done the more obvious things [i.e those that involve sitting at a piano or reading / watching stuff about playing it] and they have failed.
I find it highly unlikely for you to suggest for 'nurture' that the things which may have triggered your musical interest are directly musical in that obvious way. It seems to me that lots of people go to piano lessons and hear music and have similar things like that happen to them, and their abilities and interests vary.
No, if nurture affects and the environment does it's job, then I would say that it must do so in a way that is infinitely more complex than something obvioius like the idea that my grandad farted in Bb a lot so I grew up to like a particular piece of music or to play the Saxophone well. It's too neat.
So it could be Where their pram was put? The weather? What they ate? The chemical composition of the water coming out the taps in their house? Illnesses they did / didn't get...and possibly a bazillion other seemingly unrelated things...stuff that to all intents and purposes would be chance or random.
The fact is, no matter how much I convince myself that I could have been the best pianist in the world if I'd wanted to be, it's not going to be true and it's not making me any better. Has 'nuture' stopped for me at 38? Why should it? I'm still existing in the environment, but what magic thing can I do to improve piano? It can't be 'play the piano' because like countless other people, all of whom are at different levels with different abilities, we're all doing those direct things, so what's the magic 'nuture' for outstanding tennis players, accountants or noble prize winning scientists?
From which I'd say that the 'nurture' argument, even if it were completely true, doesn't actually help you do anything, except perhaps some might feel better in themselves, moreso if they still fancy the idea that they can do something about it. At what point does that someone who isn't as good as Hamelin but carried on trying thinking that they could do something about it decide that 'nurture' has let them down too?
If you still can't create world class pianists or tennis players using 'nurture' more efficiently than currently, or if you can't create your kids so one is 5' 5", another is 6', the next 6' 5" and so on, so you can walk up them to reach the roof, then, imo, you've neither proven the nurture argument nor, more importantly, you haven't actually made it useful.